Didn‘t read through the entire thread, but found „Peak Torque‘s“ Video on power on Fully bikes interesting. The power meter might show the same power for a locked out suspension and a open suspension, but you are climbing at different rates. The open suspension has the power meter over read… for more insight, it’s better to just watch his video:
Sorry if this had been discussed earlier.
So, additionally to the more upright position (and maybe better physical adaptation to riding that position), the PM on the MTB might also be playing tricks.