I've been blasted twice by the Rreign and usually the ash markings on my HA go away pretty quickly. This time it seems to be stuck though even before I was burnt a second time. Is there a cooldown usually or was it a glitch when it went away almost immediately?

Edit: Right after this it went away. This can be locked now I guess.

there isn't a good answer to this because it is not just Subeta dependent but also depends on your browser. If the ash sticks around longer than you want try clearing out your cache and cookies.