I had this happen once recently. We were fighting the troll; I think it may have been in mid attack when we brought it below 50%, but it actually took a few more hits before (bringing it somewhere <40% health) before the regeneration animation kicked in. At which point, the red bar jumped up by about 10% and the orange bar filled it the rest of the way. What was odd about this – in the period between when we got the troll to <50%, hp; we landed a few hits on it, and it went through a few movements and attacks.
Again, what was odd about this was not the regen as expected, but rather that the troll kept acting normally for ten seconds or so after we had pushed it below 50%.