I picked a young player in the youth center, changed his training and now his initial 300% training boost has disappeared.
It's a bug?

No its not, changing training wipes all the training progress.

You also get a warning when you change training above 100%, alerting you about the reset. You must have pressed OK as well...

There was no warning in this case.

If i try to change training to a player in my team, i get the warning but i've got nothing for the young player with the training boost.