Vo2max down the t.... Last two weeks?

I think there's a lag behind heat acclimation and VO2 max as mine has dropped two points in this warm weather. I find a dose of intervals gies me a good idea so tomorrow I may do some and see how it goes. (HRM-Run user as well)

This happened here too, but I am showing a "heat acclimation" which my 5x doesn't show at all.  I lost two points of VO2 but my times have not changed materially nor has my max HR during workouts.

I suspect there's a delay in the algorithm involved here which was not a factor with the 5x, but is with the "more accurate" 6x algorithm.  We'll see if I get it back over the next couple of weeks.  Times and subjective effort say I haven't actually HAD a change in my VO2..

Heat accilamation is not a factor in the calculation unless the temperature is above 72F. You said the temps in Quebec have gone up, but a quick glance of the weather online in Montreal shows mostly highs in the 70's. So if your body is acclimated to, say, 50F, and suddenly the temp is 71F, your body will be less acclimated to the new higher temps but the watch won't account for it.  If you went for a run when it was, say, 75 degrees outside, then the heat acclimation offset would be there, but would be very minor compared to, say, running when it's 100F outside. Basically, the higher the temp, the faster you acclimate to heat and the more dramatic the offset for your training stats.

The short version is, if you keep running in the higher temps, your VO2 max will correct itself after a short time.

The Garmin VO2 MAX is only indicated how hard your last workout was, if you want your VO2MAX steady no drop, you need a long term solid endurance base training!