So this known issue:
In Training Mode, if you have a controller in any other port besides Port 1 and enter Training with that controller, you will be able to control when the CPU shields. (e.g Controller in Port 2, enter Training Mode, when you shield the CPU will shield also despite being set to Stand)
What's going on is there is a mixup in the lightshield input.
If you enter training mode as anything other than P1, the game sets you to P1, (it swaps your inputs back to P1, and over your characters head a P1 displays.) Every input gets mapped back to your P1 character, except for lightshield/partial shoulder button presses--which are recognized for the controller port the controller is actually in.)
You can only shield with the one computer your controller port is actually in (so in P3, you shield only with the second computer on lightshield.) You cannot airdodge with the computers (full presses are registered correctly.) You can't Lcancel with a light press with the human character--the corresponding CPU just lightshields. Similarly, set the computer to "Control" and the port 1 computer will lightshield for the human character.
Seems like just a matter of identifying the code that swaps it over for the other inputs and duplicating it for the new partial press. Also y'all probably knew these details already, or definitely would have been able to figure them out quickly.