I remember taking such care to document my problem with screenshots and don't know why they didn't end up in my question, but it sounds like the same problem.
I don't know if it's relevant, but my materials were copied from a previous course in which they functioned as expected.
And mine only happened when I had two items in the module (although i only used the feature for test & retest assignments, and put those in a single module, so the misbehaving modules just happened to have two things in them)
I remember recreating the misbehaving module from scratch and re-editing all the requirements and assuring the assignments in the module were in the correct order, etc. (and documenting the whole thing with screenshots) and it didn't matter.
What fixed it, if I remember correctly, was adding a new thing to the beginning of the module (some random page), which got it to work as expected, and then removing that addition, and the order was still enforced.
At the time I thought the fix was adding a third item. Maybe it was just adding a new item?
I wouldn't be at all surprised if that doesn't work for you, but it's an easy thing to try.
Good luck!