I've had this issue with 2 or 3 players now where I've used the "move to club" feature within the editor.
The player can play in games for me, he is contracted to me, however, he doesn't show at my club and can't be trained by my club.
Attached is a player I moved when he was 16, he's been playing in matches for me but it is counting on his history as he's still at Bayern. the 2023/2024 stats are actually from my games not for Bayern. I have tried giving him a new contract which you can see in the contract image but that hasn't worked.