FL that first had FLEX I think, if not that one then a recentish one. What happens is that when a song is saved as a new file (not saved to an existing file) focus is lost on the fl ...
leave it at that for now. If you need more information, let me know.
Serum version: 1.21b4 (the issue also happens with the latest pre-release 1.22b1a1)
FLStudio version: 20.0.4 [build 629]
OS: Windows 10 1803
Everything 64-bit ...