Joplin does not remember the windows size I prefer on start up. I have to manually resize the window every time I restart the PC. It fine, when I close the window and restart the application, but the moment I restart the PC, it does not remember the last window size and position. While this a minor inconvenience, it is still somewhat annoying and a bug.
Is it possible, that it has to do with my monitor? I am using a 49" Samsung G9 Neo. Maybe the 32:9 format is a problem for Joplin?
I am another user not a developer but I have tried to duplicate what you are seeing and I have managed it under certain circumstances.
Joplin stores the window size and position in a file called window-state-prod.json and from my testing it seems that the window settings are stored when Joplin is quit (File > Quit). However if Joplin is running when the system is shut down it appears that window-state-prod.json does not get updated. So when you open Joplin after a reboot it reverts to the size that is stored in window-state-prod.json and not what it was when you last used Joplin.
As a work-around try setting the Joplin window to where you want it and what size. Then Fully Quit Joplin (File > Quit or ctrl+Q or right-click the tray icon & select Quit) so window-state-prod.json gets updated. Hopefully after that even if you reboot / shut down when Joplin is running the window will open at the size and position you want it to the next time Joplin is used.