A nice lady from MM USA directed me to send them a feedback explaining what the problem is.
Let's see what happens next. Having my boarding passes with UA number on them I have some hopes.

Do not want to remember how much I struggled to make my online checkin with seats dissapearing and seat differences between my FTL account and online checking website. The good thing that I got finally the best seat on A380 in Y, but I consumed a lot of energy for that...