This seems to be quite random,  The chunks on my experimental servers, which both have quite a few mods.. Completely different mods from eachother though, none of the same on either server. Whenever mobs spawn in specific chunks, or items are dropped.. They just freeze. I'd also have to mention that redstone doesn't work in these broken chunks either. It's almost as if the chunk is unloaded, permanently. There is no possible way to fix this from what I've seen. It's been completely ruining my servers.

*https://youtu.be/U4gsVLR7WZs*