Navigation Rerouting - bit rubbish, or am I missing something?

Not sure why you got a downvote. This sounds to be very much the case:
https://www.dcrainmaker.com/2020/06/garmin-edge-1030-plus-in-depth-review.html/

Now if you go off-course it’ll warn you within usually about 3-5 seconds depending on your speed. However, this is where one of the changes is on the Edge 1030 Plus – the new re-routing and pause navigation options. Once you go off-course, you’ll get three new ‘Re-routing’ options:

A) Re-join where you left the course
B) Skip ahead to the next logical point to re-join course
C) Cut across the course to somewhere way downstream

How each of these reacts will depend entirely on your course and where you are. For example, on my ride this morning (which was a wonky lollipop route), I made a purposeful route diversion in the first 60 seconds. The three options thus were quite drastically different, with the rejoin/skip being spot-on as expected, but the ‘Cut Across’ option basically said ‘Let’s call it a day and go home’. And you can’t really fault Garmin here, it’s doing exactly what it says – cut the course (useful on a much longer course when you just need to get home).

No news yet as to whether this will make it to the 530 / 830 / 1030 though...

I am glad to hear that they're working on a solution. However, how is it my car and my phone are easily able to re-route me (and have been for many years) and Garmin are still struggling, and developing overly complicated solutions for an ancient problem? Does not add up in my world. I think the TomTom app on my iPhone 3G was able to do this... SMH

In your car you don't usually plan the route ahead of time, you just select the destination.  Therefore you don't really notice any re routing as there is no planned route to deviate from.  Garmin head units already work this way. if you just tell them to generate a route to a destination.

Exactly.   You create a route to go *exactly* where you say -- up this climb, avoid this busy street, go around this loop three times -- so there's no way for the Edge to know the correct solution when you go "off course".   Are you intentionally diverting or did you just screw up and you really want to go back and do that climb or segment?   Contrast that with route to *destination* which is what your car does -- you only care about getting where you want to go (as quickly as possible).

It isn't clear whether these new 1030Plus re-routing options will be coming to the 830, but that would be added flexibility for these off-course options.  Also, the Pause Routing would be useful when you are intentionally deviating.

When I go from A to B in my car - how is that not the same? It is pre-planned just the same, even if it happens just a few minutes in advance... if I deviate from the planned route, a re-planning is initiated.

My Garmin can't even handle A to B re-routing in a reasonable way (hit or miss). I understand that a loop is a slightly different scenario - no rocket science though. 

Following a course is nothing like the navigation you typically do in your car.  If you have a Garmin Car GPS, connect it BaseCamp and load a pre-calculated route onto it.  Follow that in your car and you will see that it will behave completely differently to the normal A-B nav that most folks use.

This is a bit of a simplified description.

When following a course there can be hundreds/thousands of shaping points that basically dictate to the Edge where it should take you.  These shaping points are read by the Edge (the darker "purple" line), combined with the map data that it has built in and a new route is calculated (the light "magenta" line) that does its best to follow all of the shaping points from your route.  Mostly this works well for folks.  There are some oddities that happen now and again like short cuts and the like but generally this method works fine is what most people use their Edge devices for.

Compare this to A-B nav where there are basically two shaping points.  Where you are now (A) and where you want to go (B).  Its then up to the Edge to calculate the route based its map data and profile settings, avoidance etc.  If you divert off the route, no bother.  There is a new A, the B is the same so I’ll create a new route for you.  There are no intermediate via points or shaping points that it must absolutely take you through, just B.  Get me to B.

When you are following a course and divert off the Edge tries to get you back on route.  The old, old, old way it would do this when you had auto-recalculate on was to basically plot a new route to your end destination.  That was laughable and not what people wanted or expected the behaviour to be.  So it was ‘improved’ a few years back to attempt to predict a point on your route to get you back to.

Maybe this is how they map out to those 3 options on the 1030 plus.

Rejoin - exactly where you left the course.

Skip ahead - best guess

Cut across - ok I'm lost just take me home

I don’t know, I’ve not got one (yet ) so can’t test.

With all of these situations this is where the Edge switches from following the course to A-B routing, with A being where you are now and B being one of those points on the route.  A new route is calculated using the profile, map, avoidances, prefs, popularity etc that come with it.  You can see this happen because the time to destination, distance to destination all change to reflect the re-route.  Once you are back on the course it switches back to following the course for you.

Given all the *** that these things have to put up with you have to give the little units some credit.  They do not do a bad job and like any GPS Nav device who's never argued about the correct route to take?

However.

In this particular situation and in many others I've experienced myself and seen here on the forums the units do seem to struggle to make A-B routing decisions that most folks end up being happy with.  Even Garmin’s own Course creator takes the Minor road and not the Tertiary one.

This stuff is hard to get right so that it suits everyone.  Sometimes it’s the source map data, sometimes its how Garmin is taking this map data and translating it into its own IMG map format (MKGMAP), sometimes it’s the routing engine, sometimes it’s a combination of all of the above.

We don’t know how Garmin translate the OSM data into its own Maps because that’s not public.  The other maps I’ve linked above do publish their Style files on GiHub so we can see what is happening.  What we do know is that the other OSM maps take the expected route, Garmin’s own route plotting software takes the expected route yet their Cycle Map means that the Edge takes the Long way around as for whatever reason it thinks its faster.

Ok - I understand where you're coming from. I was just assuming that Garmin is doing some clever analysis/abstraction of the breadcrumbs in a route in the minute or so it takes to calculate when starting a ride... maybe not.

I guess it is rocket science then. 

Maybe they should not advertise the feature if it doesn't work (yet)? 

Locus map slightly different solution and result.

Use fast forward button to skip uninteresting parts.

https://youtu.be/p3Q6lwydYHE