I believe it is either caused by incompatibility on the graphics side with Ren’Py.This is unlikely, IMO, unless you approached the overlay differently than other people have, as I don’t see the same artifact in their animations, and they certainly use animated overlays and containing alpha.
Alternately, I noticed that this issue was emergent; for the first 6 or 7 onions it was fine, then started to be an issue. This points to it being a garbage collection or memory issue. Thinking back through the use leading up to the behavior, I did review the onion images a number of times. Moreover, when it started to happen on the 8th onion, I scrolled back before the onion to the phone and the onion was still flashing and animating which seems to indicate a possible scripting issue, I think with triggered events.