Yup - same here since around a month or so.
Just received an update today and it’s still not fixed.
Having the same issue
Do you have the same issue if you follow the OP’s steps to reproduce it?
Provide extra information to complete the original description of the issue:
If relevant, provide additional screenshots/video:
Just to add, same for me…
When is Sim update 15 slated for release? June? It’s not listed on the roadmap. I have the same issue but current prod build is 1.36.2 and the post at the top says fixed in Sim update 15 beta build 1.37
Sim Update 15 is tentatively targeted for April 30.
Do you have the same issue if you follow the OP’s steps to reproduce it?
Provide extra information to complete the original description of the issue:
Same problem.Will not open in full-screen. Reverts to windowed.
If relevant, provide additional screenshots/video:
Hello @bmwRacer50,
The current Development Roadmap shows a tentatively planned release date of April 30 for Sim Update 15. As always, this is a target date only and could change if there are any release-blocking issues discovered during the ongoing SU_15 public beta test.
Thanks,
MSFS Team
1 Like
Really! This is your solution MICROSOFT, Really!
Is there any other games, “simulators” out there that has so many problems?
FSX had better ATC , Better weather system, Better Ai traffic and Force feedback, thanks MSFS 2020 for the better graphics, point.
Considering you have to press three keys to sign in to Windows I don’t see pressing two much of a chore.
I spent more time with X-Plane than P3D, but yes, they had more than their fair share of problems. You should have seen what the XP 11.5 Vulkan beta was like.
With me after the last beta update (1.37.12.0) the sim go’s to windowed mode.
So i think it is still not fixed. 
If true, that’s not good news. Do you still see the brief appearance of the screen that indicates an update is ready to download? That’s the thing that triggers the windowed mode I think.
I see a short flash of the screen that indicates an update is ready to download.
It’s less then a half second.
Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes
Provide extra information to complete the original description of the issue:
Every time I boot the game, it reverts to windowed.
If relevant, provide additional screenshots/video:
this is happening to me too. is it everyone? how do they not catch this?
Well that’s interesting. I thought it had been released.