Users on the mailing list report toolbar buttons intermittently being painted with unwanted borders (see "[11.3 FlatLaf] Weird Toolbar Icon" email thread). I have attached their screenshots here.
Bilu Al: "this happens sometimes like 2 times out of 5 at the start. And sometimes when restarting it is back again at normal without doing anything"
Juan Algaba: "I have the same problem 7 times out of 10."