Everlasting Ice Forts!

So here's what I did:

In a room, hunting, have a fissure going. I cast fortress of ice, bring in a couple celpeze with me. I then cast another fissure inside the fortress (I was trying to see what would happene the the fortress melted and the two fissures collided -- which appears to be nothing anymore which is a little sad). I then leave the ice fort, leaving the celpeze and fortress inside.

The fortress won't melt. I mean, it gave the melts away message, but is still in the room. This only appears to be happening with a fissure in the room, a fissure in the fort, and critters in the fort as well. Was able to duplicate it with all those conditions, but not only two of the three.