I promoted DL coach, Ai signed replacement same week...

Does anyone know what happened there?
It doesn't seem right to me

It won't allow you to go without a coach during the season. You either need to be proactive and sign the replacement as well or replace the AI coach with whom you want. If during the off-season, I believe you have until Preseason game 1 to sign the coach.

Oh ok, I had no idea the positions couldn't be empty. Thanks.