It happened to me from time to time, most recently today (about 5 hours ago). I just pushed through and in the end WK recognised the review as being thoroughly done, without any side effects. It’s a bit annoying that the shadowing is sometimes too dark to read the screen but otherwise it seems to be a relatively harmless (albeit irritating) bug.
To @Viet: this is a problem that we can’t seem to consistently reproduce. It has so far happened to me very rarely that I never bothered to investigate (turn on/off scripts). When it happens on mobile (I use Android and Firefox Mobile) I usually assume it’s just patchy reception. When it happens on desktop (I currently use the newest Firefox on Ubuntu 18.04) it’s less clear what happened. I can rule out internet connection problem because other sites are not affected.