I discovered this as I was trying to make an enderman farm on my survival world. When I spawned in the endermite, and built the platform for the endermen to spawn on, they would not attack the endermite. I thought this was perhaps because I'd built the farm wrong, so I tested it out on a different world in creative. However, I spawned in the enderman, then the endermite, and the enderman had no issue with the endermite whatsoever. It didn't even open its mouth like they do when they're hostile. I'm not sure if this is a bug or if this feature has been removed in 1.16.1, so if anyone has a solution please let me know!