About 10+ Mobs spawn at a single Block even if the lightlevel is greater than 7.

It's currently not reproducable. It just happens from time to time. It happens at multiple Chunks, not only the one shown in the attachment.

Even 1 Zombie got "stuck" and didn't do anything, I couldn't even interact with the mob. (see the attached Video)

 

Just let me know if I can provide you any further Informations.