FSRS "Optimize" shows review # in progress indicator incorrectly

It shows “x% of y reviews”, and the number of reviews is not constant. It may at least sometimes be the number from a previous optimization in a different preset. It may also be affecting the percentage, but I couldn’t reproduce this issue at all until I tested with different presets — or maybe the number being too similar for the first two or three was it. It could be happening only with the larger quantities (18k, 35k, 55k) or when the computer is busy. Sometimes optimization ends with the percentage showing something like 80%.