hmmm… then no idea why the fps lowered…
The only thing you can do, is to check whether somewhat was auto-updated in background ( windows update, nvidia drivers, … ).
Otherwise, as say’d… just ignore the fps… ( but yes, the “why happens” whould be realy interessting )
I have one WITH fps counter ( I wrote it as link and not in-place video to keep the posts small )
: https://www.youtube.com/watch?v=xF-BcY4yHc4
Same for Xplane
: https://www.youtube.com/watch?v=Md3R5moMQD0
But as you say’d ( if I understood correctly ) and I say since long time: FPS is not all. More important is “no microstuttering”. E.g. this “smooth-feeling” coming into XPlane11 with vulcan was ‘nice’. But MSFS is not so worst as all told… in special compared with level of details and that we currently on the old DX11 engine.