I have my own version of the EC firmware that fixes these problems.
See this thread for details:
You may have a temporary work around with your current EC firmware by typing
sudo ectool chargecontrol idle
That other thread explains how to download and install the ectool.
The problem is buggy EC firmware, where it thinks it should try to recharge your battery, instead of using the battery to help power the dGPU, if the PSU is not large enough.
the “IDLE” mode means provide all the power from the AC PSU, and if it needs more power, draw that from the battery.
I have not seen a mode in the EC firmware that could handle the “Provide all the power form the AC PSU, and don’t draw anything from the battery.”