Dominique, many thanks, that was a full-scale investigation! (A note about the making: Due to the huge time difference he was up during some ungodly hours.)
The short message: There is a bug in Moodle!
A long description is here, a summary is here.
Now, how to move forward?
A. Your workaround: Quite inventive, I must say!
;)
Too late for me, in both courses the students are through those chained quizzes. In the first one we had to remove the restrictions. In the second one asked them to get full marks in each of the quizzes, doable since we were early in the course and broke the chain and asked the students to get full marks to each quiz before the end of the course. So not following the workaround myself but it could be useful to the others.
B. Find the tracker issue(s), there must be, this is such a gaping hole.
Is it the one I suspected earlier, MDL-75582? Created 22 August 2022, 2 1/2 years? Is in 3.11, 4.1, 4.2, 4.3, 4.4 and now in 4.5? Priority Major but assigned to nobody?
Most intriguing (subject of the tracker issue):
Activities with passing grade and only require grade completion (not passing) are displaying the wrong status
So, the bug is not only limited to the Quiz, rather common to all Moodle activities! That explains why the quiz developers are unmoved by the whole hulaboo (and we are in the wrong forum 
).
If MDL-75582 is indeed the cause, the net result of this breakdowns reported and the investigation is just a link to this discussion in that tracker issue? The HQ just have to sit tight for another 1 1/2 years and can say Moodle 4.5 is deprecated. Maybe they are already saying that 4.5 doesn't get fixes for bugs in functionality anymore?
"I've just added the ux_review_required label because we need to decide what's the expected result in the scenario described here. IMHO, I would expect the activity to be marked as "Completed" because it was not configured to take into account the passing grade
" October 2023