I had the same problem on Windows 10 with Galaxy. Quitting the game in any fashion would cause it to hang until someone killed it. Galaxy would think the game was still running, hence denying me from starting it again. Cloud sync failed and game time did not advance, which was annoying. Therefore it would be nice if someone could look into this.
However, I configured Cuphead.exe to run in Windows 8 compatibility mode, which fixed the issue for me.
Edit: Never mind; still happening, even in compatibility mode.
Post edited October 05, 2017 by LiquidAndross