It may or may not be related, but I was healing lfr and was at about 50% mana when I really just stood there in something and died. Anyway, I got a rebirth and came back with full mana. Quite the pleasant surprise.
Just did a test with my Enhancement and my Druid, what is I could understand is that mana don't change anymore when you die, so when you reincarnates the spell try to bring you to 20% but just add for the mana you had when died.
It was a bug causing us to keep mana after switching specs.
Lol, You just contradicted yourself, You said it's always been that way, but also say it was a bug causing it, Well if the bug was causing it, then it hasn't always been like that, I can't remember the last time it emptied my mana, apart from after 5.3, So henrytxin is correct, it now empties, which i found rather sad
Ok, so if you guys have a class that don't have the hybrid +300% mana pool, your mana pool emptied before and still now after 5.3. (i.e. Priests mana bar always emptied)
As for hybrids, you always retained some amount of mana after switching specs. This was probably because you gained the mana proportional to the +300% increased mana from your respective spec passive. Now it seems it does not add mana at all.
So Alekseq is not completely correct, but not completely wrong either. Mana pools did empty or not depending on which class/spec you were swtching to/from. Whether it is intentional or a bug, we will never know.
As for reincarnation giving you mana back. I tested it just now, and it does not seem to give you full mana back, at least not anymore.