TBT directions generated on Edge do not match up fully with loaded Ridewith GPS course?

I'm not sure what is going on in your case.  I lay out a ride on RWGPS using the OSM cycle map, download it as a GPX track or TCX Course, copy it to the  New Files directory on the Edge, and the Edge calculates a route that matches what I laid out on RWGPS with proper TBT directions. The TBT will not be exactly the same as what RWGPS generates for TCX Courses but is correct none the less.  Is this different from what's happening in your case?  I've done the same using the other maps available on RWGPS with similar results.

The maps that come with the unit are based on openstreetmap (OSM) maps.

To get the closest match, you should plan the route using OSM maps.

I just tested this (drawing the same course, once on the "maps" choice on RidewithGPS, once with the "OSM" choice and, unfortunately (from my vantage) you are correct. The maps choice generated a weird detour (briefly) in my Edge TBT directions; the OSM did not. Thank you for the assist. It's too bad only in the sense that I find the Google maps to be clearer and easier to route on, but I'll adjust I guess.

Separately, I note the "direct line" navigation option gives an error of needing automotive maps to route. On my prior unit (Edge Explore 1000)j, the direct option gave no error and seemed to 100% follow my loaded course. Not 100% sure it didn't ever generate its own TBT directions that varied, but it worked as a navigation option whereas it doesn't work on the 830 with 3.50. FWIW.

Using Google maps generally works ok but you have to live with the differences.

The Garmins will sometimes create shortcuts in the route. There isn't any way of preventing it.

Thanks. I may just go with Course Points and turn off TBT. Also, changing display to 3D makes it much easier on the map to see your uploaded course (although then you lose the display of your zoom factor).

My club uses RideWithGPS to host all of our ride maps.  They mostly work ok.  I find that the 830 is better in that respect than my 800 was.  At least so far.  I updated the OSM on the 800 and that helped so I'm sure there is a connection between which map you select in RWG and what you have loaded on the Garmin.  

That said, I have a route that I use for commuting that was generated from the 800.  On the 830 it came up with some weird turn indication where there is no turn.  So maybe it tries to modify the route in some way.  I will try saving the ride and converting that to a course to see if it cleans that up.

What are your settings in the 830?  RWGPS has recommended settings.  I do not think they have them for the 830 yet, but the 820 is here  https://ridewithgps.com/help/garmin-edge-820  It sounds like you are letting the 830 recalculate things.  I set my 830 with these settings and I have not noticed any problems so far.

Thanks, but you misunderstand the issue. I have all of these RidewithGPS settings, including recalculation to "off". It's the initial calculation by Garmin of the TBT settings that varies from what you've meticulously plotted. Matching up the two map sets (on your computer and on your edge) seems to be the only solution.

The settings suggested by RWGPS don't matter. They aren't used by the Garmin when using a course file.

The point of using a course file is to pick the roads you want to use.

The "avoidances" options would cause other roads to be selected. Rather than the roads you carefully chose.

The avoidances would be used if you go off course and have the unit recalculate.

But we aren't talking about going off course.

"Turn guidance" is a second route that follows the track using the roads on the map.

A recorded track can be a bit messy and not follow the roads on the map exactly

It's no surprise to have odd instructions once in a while.