If a game doesn't neatly close on its own then you won't be able to open any other game until you do a full system restart.
Some games you can run straight from the executable (when they're drm free) but then refuse via steam library or desktop shorrcut so it's clearly a steam issue and not a game issue.
Noticed this a few weeks ago with Cronos as it updated while launching then failed to launch but did launch from the exe, and then no other game would open through steam or desktop shortcuts until a full system restart.
Today trying out a few demos, Beneath had issues closing properly, triggering this same behaviour again, already had to restart my system once.
I also had Lossless Scaling open so I imagine that helped to guarantee the bug with Steam getting confused with one app failing to close but then having another open at the same time.

Not on a beta branch BTW