I've been using
Raid Interface Antidote for a while now and even though it fixed the CompactRaidFrame errors, the new nameplates introduced the SetTargetClampingInsets error, which the first replacement in the code of that addon is what causes it. After tinkering with the addon, I got frustrated with it and just disabled it.
That's when I noticed, during a world quest group, that unit frames were being added and removed just fine in combat. No errors, no copies of players that left, just fluid updates.
I may have missed a crucial fix at some point in the last month or so, or one of the popular addons I use may have finally did something, but I'm positive something happened that fixed this. I have 190 addons installed according to Curse Client and I'm amazed the raid frames aren't erroring. More testing of this may be necessary to ensure this isn't just an isolated occurrence. I also was directly using dropdown code in my Decliner addon and also recently disabled that code, so that could also have something to do with it. I've been meaning to use a dropdown library to avoid the dropdown taints.