There are only a few ways this should be able to happen. It is possible to raise your fire resistance to 100%, but I think that this can only be done with Malicious Blades or Entrenchment. In this case, you are completely immune to fire damage and it won't hurt your armor either. Invulnerability, of course, also works, but you probably would have noticed that.
If you are sure that you were taking damage and your armor wasn't, I'm not sure what to say. Perhaps you repaired the armor and forgot about it? Or there could be a strange bug that I haven't been able to reproduce yet.
If possible, providing a save file with the armor might provide some clues, but if the game is already over, then at least posting a mortem could help track down the bug by showing what equipment and traits were in play. Thanks!
Edit: Another possibility for taking no damage is the impact knockback bug. If revenants roll high enough damage, they'll knock you out of the blast zone causing you are your armor to remain unscratched. This is very unlikely except on Angel of Max Carnage.