Colossus boss made invulnerable with Glacial Bolt

Happened three times in a row, respawned at checkpoint each time. I had just put on glacial bolt in the Titan's Grotto, and tried it out on the boss. After doing about 15% of its health, I could no longer damage it. After the third time I just switched back to permafrost and it worked fine. Maybe the ice wall had something to do with it?