Yes, I did do the questions when they appeared in the schedule, though I don't remember which they were. But, I was able to reproduce the bug with different kanji :)
I listed my steps below, but in short, it seems to trigger when you completely study a kanji schedule for the day, then incorrectly answer a kanji quiz reading question in the study center for a kanji term that is in the completely studied schedule. After completing the quiz, even if you requized the incorrectly answered question and got it correct, that previously completely studied schedule will show the incorrectly answered kanji term(s) as being available to study.
Steps that triggered the bug:
1. I studied all terms available for the day in my kanji schedules 33314, 33313, and 14170.
2. In the Study Center>Kanji Kentei>準1級 section, I did a kanji quiz for the lesson Step 1 with only the readings question style enabled, with only the advanced question types Match the Readings (1-1 reading/kanji), Kanji Crosses, and Complete the Vocab (1-readings given) selected.
3. I answered a Match the Readings question incorrectly by mismatching several of the given readings with a few of the 6 given kanji (亨亙乃之伊丑). It took me several retries to finally match all of the readings to the correct kanji, and on different retries I answered different parts incorrectly/correctly. When I finally answered the question successfully and completed the quiz, the answer panel for the question showed the kunyomi mastery for 乃 as having increased to 5, and for 之 as having increased to 7, presumably after having dropped several levels from getting them wrong earlier.
4. I returned to the dashboard and found my kanji schedule 14170 suddenly had 2 terms to study for the day, kunyomi questions for the kanji 乃 and 之.
5. I studied the 2 terms, and the answer panels showed their kunyomi mastery increasing an additional level each, this time to 6 and 8 respectively.
One thing I'll mention is that of the other four kanji that appeared in the question I had to redo, 伊 and 丑 I matched correctly during each attempt at the question, 亙 is not in any kanji schedule of mine (though I did mismatch its readings at least once), and 亨 is in the only kanji schedule I don't regularly study (I mismatched its readings at least once as well). I am guessing that if I did have 亙 in a studied schedule and if I had completely studied the schedule I have 亨 in before doing this quiz, they could also have reappeared as available to study in their respective schedules.
Perhaps the bug has something to do with the recent changes to how the vectors for terms go down when answered incorrectly? Since I've never seen this problem before, it seems likely it was triggered by recent changes.