Hi, I beleive it to be the motion reprojection locking it, usually in VR when forcing on motion reprojection it will half the refresh rate, just like steam vr you can set auto or manually force to halve the headsets refresh rate.
So same would apply to WMR, setting OpenXR developer tool motion reprojection to ON will force it to half the refresh rate, so setting to let windows decide on what the headsets refresh rate is, will then cause it to choose 60Hz in flight simulator due to the performance, then motion reprojection will half that to 30.
I’m no expert and could be wrong on how it works, but this is what I have noticed and found to work.