That's not weird behavior, it is a bug if that is true.
If I may ask, was this Pro 3.1, that has a new Maplex setting "Remove ambiguous labels" that defaults to "Remove all"?
I have had issues with this new default setting, and needed to switch to "Do not remove" or "Remove within same label class" to get proper behavior similar to Pro 3.0, and reported the issues with the "Remove all" default as a potential bug in Pro 3.1 to ESRI, which is still in investigation.
I am now wondering if this issue is related, and the "Remove all" problems I had were actually caused by your observation: that the interpretation of the priority list may be inverted for that specific setting, and likely not with the other two, as I got proper result there.