Those things happen when you have more than 7 players available, like as you stated ->9.
The coach always subs out when you have more than 7.
So try using only 7 players or less.
7 players don't work, especially in blowout games...
I've tried almost everything with my 3 trainees, and only 5 players in roster works, but then you can't win in higher divisions...
if I have more than 5 players in roster, and a game is blowout win, coach always steal 1 or 2 minutes from my trainee with random sub in last 2 minutes...
if it is a close game, or defeat, 48+ for trainee, doesn't matter 6 or 9 players in roster...
it looks like BB is trying to punish you if you want to train and compete, especially if you are too good in competing (many blowout wins)...