Yep, its a really annoying bug. I have six of these cages in my alt guild bank now. I had five to start with, and I waited about six weeks after I originally found them before I submitted a ticket. I was told that the logs were too far back to see what the pets originally were to restore. So when I had this happen earlier this week to a Hippogryph Hatchling I was trying to sell on the AH, I immediately sent in a ticket. It took a bout a day, but I got it back. (Unleveled, but better than not at all.)
I also found another one in the pet tab of my personal guild bank just after I sent the ticket in. I added that info to the ticket, but they could not confirm that one. It was just sitting in the bank, as well. I had not taken it out or even moved it within the tab for a while. I am fairly certain it was just a random, green quality vendor pet, so I'm not upset about that one. I know for certain that three of my original five were TCG pets and two were children's week pets I had upgraded with stones, so losing those was annoying.
I am hanging on to the six [Pet Cage] items in my guild bank, just in case they still revert once the problem is fixed. I was told by the first GM that they probably wouldn't, but I can always hope. Either way, always note what you list on the AH and put in a ticket as soon as you notice they have changed. I ended up just adding my extra, rare TCG pets back into my journal so I don't risk losing them again.