Here's the deal conceptually MM+ is a GREAT idea, However it had a seriously flawed execution and rollout, But the May trip was the FIRST trip all my MB's which were linked to my MDE account worked, The usual scenario has been get MB's in mail, Bring to resort try to use FAIL then get some grey ones from the front desk and those would be the only MB's which worked on the trip.
Why did this happen - no clue, But any pre-reserved FP's would also vanish when we got the 'new' bands and Disney was powerless to fix.
Since you have not been there since MM+ rolled out I expect your experience will be good, It seems the lion's share of people with major issues are long term customers and AP holders who predate MM+