This is very much happening in 188.8.131.52, I first noticed the issue in 184.108.40.206 or 220.127.116.11 too.
Reinstalling 18.104.22.168 seems to resolve the issue,
It was really noticeable in battlegrounds where you're constantly pulling yourself into combat by healing other players.
I'm having an issue since this last version, in that HoTs aren't showing up immediately when going into combat. It can take up to five seconds to show up sometimes, to the point where I'm having to cast 3-5 times thinking the HoTs aren't landing.
An update on this. In a non-raid party, people seem to disappear from frame 1 at random and having show duplicates doesn't seem to matter. I don't have "hide bars for out of range" on either.
Another odd thing I've noticed which is likely related is that I have highlights for buffs on the MT frame (but not frame 1), but private tanks who are missing the buff that are incorrectly in frame 1 still get highlighted.
If I have some time this weekend I will try to reset my settings and see if I can reproduce.