掲示板 Forums - Cannot add a kanji reading to be quizzed

I use the „add/remove reading from quizzung“ feature a lot. A while ago this stopped working - sometimes. A reading once removed from quizzing cannot be added again. Instead I get a tooltip. Did the tap region get smaller? Do I keep missing it? Where‘s the selection with the small checkboxes?

Is there a workaround? Adding readings while I quiz appears to work, but actually doesn‘t. It‘s removed again the next time I see the kanji in a quiz. And it stays removed in the dictionary, which was different before, I believe.