I have reached out to our IT department in the meantime. If I recall correctly, I have traditionally used the classic quiz structure, so don't think I used the new quizzes overview, but ... that was two years ago, so not certain. At any rate, the interesting thing is that I have three tests in my 2023 course. Two of them show up in the quizzes sidebar but this test (#2) is not showing up there. It shows up in a Week module listing and students took the test without issue, but as noted it is not listed in the quizzes sidebar section, which seems odd. Test 1 does show there and I successfully copied it to my classes this semester. I was successful in copying the test in question to a module in one of the current classes, and the questions are there, but it does not appear like a quiz entry (at the top of the Canvas page) and I am unable to adjust the student input choice at all, nor is there a save button at the end.
Is it possible this is somehow coded as an assignment and not a quiz (though it does have the little rocket icon)?