For the past while I've been having new vocab terms appear in quizzes for the first time without appearing on the quiz intro page beforehand, without the terms that were on the intro page being in the quiz.
e.g. I just ran a schedule where 土石 and 白金 were introduced before the quiz, but ペンダント and 有様 were the new terms I was quizzed on instead.
It doesn't happen in every quiz or in a single schedule specifically. My best guess is that it has something to do with dismissing terms in the intro as already being known, as I noticed the issue after I started doing that more often, but I haven't had the opportunity to test if doing that reliably repoduces it.
I have had another person report this error with new kanji coming in, so it is most likely related. I have not yet have a chance to do a deep dive in order to figure out where the issue is coming up, but I expect to do so this week.
I still encountered it in 2 schedules today, but only in quizzes where I marked terms known on the intro page beforehand. "Introduce terms in random order" was turned on in the schedule settings for both.
First quiz, I only dismissed 1 term out of 2 (I currently have max 2 new terms per quiz). The term that was supposed to be replacing the dismissed term didn't appear in the quiz, but the term from the other flashcard still did.
Second quiz, I dismissed 4 terms in a row on the first flashcard and didn't touch the second one. Neither of the terms in the intro page were in the quiz.