As a few users reported and enquired about, I've had a close look on how to solve the issue about International bans, in particular for U21 players.
How things stand.
The actual problem is that if an U21 player is banned, and the Senior International team has been knocked out of the world cup, bans never decrease, as U21 matches are treated by the system as "non competitive" and therefore they are not useful for bans, and the team is treated by the engine as a whole (Senior+U21 team) and it cannot be split.
There is not a simple solution, as every solution is not "ideal". It has to be decided which is the best (or less bad...).
Solution 1: We tell the match engine that also U21 World cup is "competitive". So that bans will decrease. What may happen though is that the other way round can happen. If a nation is still in U21 WC, but the senior team has been knocked out, senior team bans will decrease even if they're only playing a friendly. That is because the system treats the team as a whole and if one of the team has a competitive match, the whole squad is treated as such. So it solves the problem we have, but is a little not realistic for 1st teams.
Solution 2: We tell the match engine that cards on U21 World Cup matches does not count like it happens for friendlies. That might be not ideal as well, as it would mean that there won't be any U21 bans ever and is not really realistic, but on the other side it prevents the actual problem and the 1st team is not affected like it happens in solution 1.
Solution 3: Everything stays as it is now, as you don't feel the actual system is a problem.