I also do have the Flux S and after the update firmaware last sunday, I do have much more resistance than usual. Normally I can cycle 200 watts (towards Treshold) for an hour or so and after the update…
I've had the same problem kind of - in a lower gear the power is over reported by the Tacx by around 15-20W
This is a thread I created on TrainerRoad: https://www.trainerroad.com/forum/t/odd-power…
I have contacted Garmin about this and also did a few tests they wanted me to do for comparison. The values did show did show a significant separation of power values the higher watts I was pushing (when I looked att the values in a similar graph as above) but when I got my response back from Garmin I they said the opposite; That the powers meter and the trainer was more on par with each other the higher power I had. This is REALLY strange. I will contact them again to make sure they have compared the correct files.
Any progress from your side?
I also do have the Flux S and after the update firmaware last sunday, I do have much more resistance than usual. Normally I can cycle 200 watts (towards Treshold) for an hour or so and after the update I bearly reach 150 watts (average). I contacted Garmin (Holland) and they told me there was ONE identical case like me (because of the update) and the only thing I can do is exchange mine for a repair. So my issue is like yours (Tacx do not show the watts like it should be) but also a resistance problem. I have to ride a much higher gear to reach the same watts.
I downgrade the firmware to 3.3.40/1.1.12 (I did have 3.3.40/1.1.16) because of the problems with the resistance I could hardly cycle but unfortuneately it made not a big change.
But I am certain the output Tacx Flux S gives is NOT the rigt one. I can feel my legs and they don't lie.