I think I was just reproducing it successfully (did it 2 or 3 times in a row):
From nearby and in sight, I targetted my enemy then targetted my pet. Then I ran out of LOS down a ramp a ways (I was still in casting range if I had had LOS, I believe) and used the "switch between last two targets key." This bugged the enemy's hit point bar.
The only way I could get the HP to refresh was to move back into LOS, escape to clear target, then retarget with tab or a click. No combo of toggling between last target or /assisting my pet or anything updated HP until after I moved into sight of the enemy and cleared target and retargetted manually.
Edit after more fiddling - definite seems to be connected to line of sight. Looks like I didn't even need to escape to clear target on my most recent fight, but I did have to be in line of sight when toggling between targets or else HP wouldn't update. And then even when I move into range, HP wouldn't update on their own... I'd have to flick between targets for it to refresh.
Edit 2 - only switching off of a target while it is out of LOS seems to start the issue. HP were just updating fine with a target out of LOS, but as soon as I switched targets then switched back, HP bar was frozen in place.
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