I also noticed that problem lately. That usually happens only when some player is d/c out of range or something like that. For some reason tags with 'UNIT_HEALTH' and 'UNIT_NAME_UPDATE' events do not properly update.
It's pretty easy to fix though, just force frequent update on the text region with that tag, e.g.: n.frequentUpdates = 0.5 (0.5 should be enough and not too much to affect unit frames' performance)
I guess it's not the best way (sort of hackish) to do this performance wise, but so far it's the only one I managed to figure out to get rid of that weird behavior.
EDIT: or wait actually never mind ^^
just noticed another topic about this issue:
http://www.wowinterface.com/forums/s...ad.php?t=36869
so I guess updating TagEvents to 'UNIT_NAME_UPDATE UNIT_CONNECTION' should help
