Hmm yeah that’s very odd. I’m going to keep an eye out on my end for this issue and if I find it, I’ll try to find a solution.
Since I can’t currently replicate it on my end, this is a tough problem to help solve, as it’s definitely not expected behavior. My one suggestion is to check that “Use as Occluder” is turned on for your blocking mesh, though admittedly I see no reason this would be turned off. You could also try turning off Hierarchical Z-Buffer Occlusion if that’s enabled (with r.HZBOcclusion=0), since this culling method isn’t as accurate.
That being said, it seems that other people have had this issue too
Unfortunately, it seems like no one there had much luck either. You could try using Precomputed Visibility Volumes if you’re on 5.2 or lower, or use one of the suggested plugins to try restoring UE4’s Software Occlusion Culling.
Given the amount of complaints I’ve seen about this and similar issues, I’m going to see if I can raise it as a concern to Epic for some more in-depth documentation. Good luck with this! I’ll keep an eye out for anything else that might help.