I have the same issue as well and want to tell my story which may have some relation with the issue.
Few days ago, on the day when the group stage started, I won the match just a couple minutes before the start of the group stage. I was offered with three free players for my team but I didn't manage to choose any because the group stage actually started and that proposition with three players disappeared. I asked the support if it was a bug (because I felt like it was so), but the support answered that they can to nothing. I still believe it was not fair because there were no warnings that those free players will disappear when the group stage starts, moreover there is still an option to purchase high-starred players for tokens.