Gotcha. I didn't really bubble spam that much in Wrath (I know it was effective, but I found it boring), but all that mana back on Infest was too good to pass up. I guess I figured it was intended, didn't realize it was a bug.
I believe it was intended in Wrath, but it came back again out of nowhere in MoP Beta. For all of Cataclysm, Rapture was one shield break, then a percentage of mana returned. There was no gaming it, there were no multiple returns. In MoP, for no apparent reason at all, Rapture as it was in ICC came back.
I noticed it on Heroic Nefarion before T14 had been released, we were just joking around with our fresh 90's. I blanketed PW:S before an electrocute and had nearly OOM'd myself, the instant the attack triggered and my shields broke, I was at full mana. It was easily able to be replicated on fights in T14, especially Stone Guards, Elegon, Garalon, etc. Sadly, it was the only way to be competitive at Disc in 5.0 launch, and most Priests had not figured it out either. I don't think it was fixed until Disc got the overbuffs of 5.1.