Difference between ‘Run’ profile and ‘Trail Run’ profile.

Trail runs are not used in calculating vo2 max as you mentioned. 

This 1st post explains it well

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-935/128780/run-vs-trail-run

I use "Run" for my less technical / hilly trail runs.

Thanks for pointing this out !
Interesting thing that trail runs aren't registered for VO2max calculation.
I am quite often going for runs with my wife (which runs a bit slower than I do) and usually my VO2max estimations drop after we go for shorter, slower runs together. I guess using the "Trail Run" profile would be a nice way of avoiding this ! What I'm wondering then is if Trail Runs are registered for training load ? Because I would still like these "slow" runs to be accounted for base / aerobic training...

They do still provide training load.

That normally shouldn't happen because your heart rate should be lower on those runs. The reason it's necessary on trail runs is that the technical features of trails can make the aerobic load higher than you'd expect based on just the elevation profile and speed, and the watch has no way of knowing that you're clambering over rocks and roots. 

I think the algorithm for VO2 max estimation is poor. My VO2 max fluctuates between 60-64. It seems to move up one when I do a tempo run and down one when I do a recovery run the following day. Does that mean the watch thinks my HR should be lower for a slow pace run? It's stupid because you don't loose fitness in two days. Also some people just run with high HR. My average HR in a half marathon is 182, which is about 92% of my max HR, which may seem like I'm unfit but my half marathon time is 1hr 23 which is still quicker than your average male runner in my age group. I could then do a recovery run the next day at 8 min/mile and my HR will still be 170+.

To be fair that algorithm is trying to account for the middle of the bell curves using just a few data attributes.  This generalization makes it more difficult to account for outliers like yourself with a normal HR of 170+ for a recovery run.  My own recovery runs are around 110 BPM in 90F and my half PR is 140-145 at sub 50F temperatures.  My RHR is 41 with dips into the 30's.   The point of that is that trying to account for those extremes is tough. 

With the 6 it's finally supposed to be taking in account heat, that's one complaint I've had for the last few years since the 3HR is watching my estimated VO2 go down during the summer and then back up during the winter even though I run the same but the heat causes a several degree shift in my average HR for any given effort. 

But even with that, on a week to week basis it's always been very stable with shifts of only 1 or 2 point between the random times I've happened to check it. 

Also it's very much just a approximation based on extrapolating data from actual VO2 laboratory tests.  It'll be in the ballpark but the only way to get a 100% accurate value is to get it tested in a lab. 

Hi, I use trail, and get vo2 and performance condition? Has this changed more recently? 

Yes. They’ve tweaked the algorithm to take gradient and what the watch picks up about surface into account. If it isn’t working well for you, turn off Record VO2 Max in the trail and ultra run settings, where it is now switched on by default.