First, I have the A310 and never have this problem, as its my most flown plane in MSFS.
I have setup the throttle as its own profile for the A300 and have done the throttle calibration multiple times. The issue seems to be the engines going down to 0 in the Idle position once starter/APU is off. I can keep the engines going with a bit of throttle, but it means I cant bring the plane to Idle when landing as it dies on the rollout.
I still have not seen a workaround here, so not sure if anyone else is having this issue?