Do you mean he does not become invisible at all or becomes invisible, but continues casting so to reengage again? If it is the latter, this has happened pre-4.2 as well, you can add /petpassive into your invisible-macro, which will fix this behaviour. I used that macro before and after 4.2 and did not notice any difference, but I haven't raided as Frost lately, since I found it easier to get 45k+ hits on Shannox's add with arcane spec.
I'll have to check tonight. I've got him set on Assist, so I'll try it with both that and Defensive stance and see if that makes any difference.
Agree on Arcane being a better choice for breaking Face Rage, especially with the CD on DF being 30 seconds and Face Rage being called every 26-27 seconds. However, I tried Arcane and almost fell asleep, and we've already got one Arcane mage in our raids, so I'm all about diversifying.