Per MC-134974, My wife, a friend and I are in a slime chunk. We couldn't see something, but it appeared to be attacking us. We realized the "attack" would happen when we entered certain blocks. We couldn't place sand on the "invisible attack" blocks, but we could drop sand. After mining up the sand, we would continue to be "attacked" entering the same area. Attempts to set all entities glowing did not reveal these invisible "attackers".

When we rolled back to 1.13.0, those same areas had living small slimes. Searching, we found MC-134974 but no reference to "invisible" entities. This issue may be specific to slimes and not other entities. If appropriate, we can attempt to duplicate this and send a worldfile, but this may be resolved with fixing MC-134974.