The whole nameplate randomly hiding issue seems to be related to the new "spread nameplates" feature. I do believe this because there are the following console commands that can "fix" the oddly behaviour.
1) /console bloattest 1
2) /console spreadnameplates 0
If you only use #1 you will end up with literally bloated plates, but they hide/show correctly. If you also use #2, there won't be much of a difference. BUT when I did this:
/console bloattest 1
/console spreadnameplates 0
/console bloattest 0
Everything was working again, no bloated plates, hiding/showing correctly. However I had no time to "stress test" it.
Edit: It's broken again after a relog, though.... D'OH! It's possible to repeat the process on each login ...
Edit2: The behaviour seems completely erratic, after login/relog/zoning and repeating the procedure, mentioned above. Sometimes it works fine, most of the time it doesn't. -.-