Yes, it's a new semester and it's precisely what I was trying to do, set the grouping. There are 12 assignments submitted in groups, and I had no trouble setting (most of) the others.
It turns out, there were finally 2/12 assignments with this problem. They are in a copy of my other course I set up this semester that I had this trouble with. Nothing has started, so no chance there was anything submitted.
The workaround I did because courses start tomorrow was to duplicate the bugged assignments and rename. The duplicates were fine (I could configure them as I liked).
I have kept the "corrupt" originals and hidden them from students in case someone wants to debug more. I know I had erratic problems in the past with assignments/quizzes becoming "corrupt" (strange behavior) in copied courses, and the solution was to duplicate them. It's non-deterministic, and I usually have so much to do at the start of the semester (a Moodle click-fest) that asking in the forum is low-priority.
Another quirk is that our site occasionally gets "database
write errors" (it happens infrequently, but since I have to edit a lot of items at the start of a new semester, with 12+ quizzes and 12+ assignments, it has happened a few times in the past 3-5 years). Who knows, maybe the write errors happen during the script that is copying the courses, and some items are improperly configured? I am assuming the operation to copy a course is not atomic in the database
, but I have no idea really.
Thanks all for the responses.