I'm seeing this as well on my well-equipped characters. On my untwinked newbies this doesnt seem to be an issue. So perhaps the +hp from items isn't getting factored in correctly.
Interesting to note, when my roommate and I are grouped, we each see the other's hp% dropping normally, but our own generally stays fixed at 100% until its too late - and then it only dips to maybe 70% before were dead. (I haven't done any parsing to see if its actually the correct %, or the % without +hp gear, but at least we can shout out when the other person is about to die
Also interesting to note, when the health bar is obviously showing the wrong value, doing a #showstats on yourself seems to show a more accurate current hp.
-nattini