So, with my guild and with pugs I have already run ToC10 and 25 and in all tries the elite adds (the big ones) do burrow when they are on an ice patch. That means that they have to be bursted down immediately, requiring much more DPS and making it nearly impossible if you are not overgeared. This applies to 25man normal mostly, since the hp the adds have in 10man normal is not that much.
I know the instance, as all others, are buffed. That's ok. But it is for that reason that this mechanic has to work correctly. If not, it becomes nearly impossible.
I confirm the bug, another aproach that u can use instead of pure burst is that their burrow is interuptable by some spells (some work, some don't). I know that Paladin's holy wrath and Shaman's thunderstorm do the trick for sure.