This has been in feature requests for some time (actually it’s archived now), but it’s really a bug. The window remembers the size of the window prior, but it doesn’t go into full screen mode. It’s literally one call…
func toggleFullScreen(_ sender: Any?)
So when someone goes into fullscreen, store a bit in a variable in the preferences and when the app opens if that bit is true, run that one call and eliminate an annoyance for virtually every Mac user.