I found the following post about a bug that seems to be the same. (I’d searched for similar bugs, but focused on hyphenation and ligatures, so didn’t find it even though it was right there below. Oh well.)
https://forum.literatureandlatte.com/t/strange-character-substitutions-in-full-screen-mode/14207/1
I tried the newest beta (2.1.0.5 15145) and it did not help.