I am sorry if this question had been already answered somewhere. On some occasions I noticed that my armor and maybe even a shield card has red or white dice icon attached. I suppose those are some modifiers but I am not sure about that. Here is an example Please if you have a spare moment to explain this to me I would appreciate it.
That is what you rolled the last time the card activated. It's something that has only recently started showing up. It seems to happen most often when you quickly click through an animation. So, in this case, I would expect that the last time your Rusty Armor tried to block damage you rolled a 1.
It seems to be a long-lasting bug and I hope it changes. It could, technically, be of help for that little die to stay visible in your hand: if you click through like crazy, and then want to go back and see what the exact roll was, you can consult the small cards. However, even if that were intended, I doubt the little die is supposed to remain there when the card is triggered in the future. You know, when it pops to the middle of the screen and there's a new roll. It's annoying to have your last die roll sit in the way while you're rolling a new die.