That happens to DD if I don't change her every two hours. Or if she is teething, and her wee is a lot stronger, then I sometimes change even more often to stop the rash.
Though she generally gets more of a sunburn type rash, not spotty.
Oh, and I started using Invisible Glove on her to protect her butt a bit more - it works well. I haven't had an issue with washing it off either, that I can tell (it's plant based)