So… some tests later… There is an issue.
Settings IN-GAME : V-Sync=On : 60 ( full )
Normal ATC Not ext. Window:
ATC now ext. Window. Get FIXED 25fps
Back to interal ATC-Window:
Thus, it seems there is another kind of V-Sync Bug, similar as the old one, but not same:
https://forums.flightsimulator.com/t/broken-v-sync-limiter/324014/14?u=michamma
If open ATC as ext. Window , the Game jump to the second V-Sync step ( note: I own a 75fps monitor… 75-37-25 ).
Not sure why in your case seemingly it jumps to the first V-Sync step ( 60-30-20 for a 60fps monitor ) … Or own you a 120 FPS Monitor ??
I also re-tested the fix of the old bug, and the old one is still fixed.
With the V-Sync-30 Setting, I get the correct first V-Sync-Step (37fps for my 75fps monitor).
Have you already created a ZenDesk Ticket ?