Microtransaction Tier??? (Locked Option)

So I was changing up my cosmetics and two items from the last mystery box now I have dropdown arrows that read "Select Microtransaction Tier" with only 1 roman numeral open, and several others greyed out. I thought it might be variations owned, but I have more than what is unlocked and the roman numerals don't add up to the number of variations that were offered with the item. Anyone have any idea what this is??

I assume it's for the rewards of this league which comes in different Tiers.

Up to this post. I also have this dilemma. I keep unlocking mtx tiers without any idea how the heck am I doing it. Someone tell us how this works please.