I've had this happen to me a on occasion as well, not exactly sure of the cause or why steam says blockland is still running when the process doesn't even show up in task manager. It's not something I'm able to reliably reproduce either, most of the time blockland closes just fine but
sometimes it will still say it's still running when it's not. only restarting steam fixes it.
It's really inconvenient when you're working on an add-on you can't update in-game and have to restart Steam every time you close the game down
right?!
The game is just actively deteriorating from neglect at this point.
I don't think it's a bug with blockland, more likely to be a bug with the steam client. I do not recall this issue appearing until recent steam updates