Happened during testing with multiple spiked mails and clicking very fast to speed the anims up. More testing reveals that its not exclusive to spiked mails only. Not sure if the die value is tagged to the corresponding armor or the last roll. Plus its not consistent, reproducible after many tries...just not consistently reproducible. Hope it helps.
I've had this happen a few times and finally had a chance to get a screenshot today. I also click through the animations to speed things up, but not sure if that is a cause. The die seems to be from the last roll, but after playing a few more rounds the image doesn't change even though I've had to roll armor checks. This doesn't see to affect future rolls and is possibly purely aesthetic.
This bug is still here and easily reproducable by fastclicking during dicethrows on armor. It also shows on the bigger armor picture when you throw the next dice for it.