ETA: looks like a neuromatch problem and not universal thanks for the reports!
When I look at the bot's (@xkcdbot ) alttext from last night it's correct on the indieweb.social page, but as seen form my instance the alttext never updated. Could someone report back from a different instance to see if it did for you?
Normally I see it update pretty much as soon as I finish refreshing the page after I finish the edits. Disappointing if it's having an issue on one I spent a lot of time on.