After reading all the posts from FT and the FB group to date, it seems that one should ask about the number of years on record as of Dec 2018 and then go from there. Previous screenshots during the merging years and the elite year calculation seem to be the disconnect that I can tell.
for example, a FB post says they should have 4 years of platinum since they got upgraded to platinum in 2017, but Marriott responded that the member only has 1 year of platinum at Dec 2018 so it is correct that they only have 3 years not including the 2021. So the year one attains platinum is not counted (which has subsequently changed based on change in terms as someone responding mentioned).
my bet is Marriott is working off of some merging data driven by certain lifetime criteria that most of the employees don’t even know; members using incorrect calculation of years (as the example shown above); Marriott mistakenly double counting matched status for certain years that resulted in years already shown during the merger and unable to adjust backwards which makes it appear like it is not moving (clawback); or a combination of all of the above.
one thing is for sure, Marriott is just patching things here and there and the external facing reps don’t know any better.