As licenced engineer on early A-320s (28 years ago) I can attest to many problems of power transfer during engine starts, resulting in ECAM warnings/faults etc. It was so common that often it was not written in ATLs at all, to be honest. Typical remedy was to reset related C/Bs, then run the test thru MCDU and worst case, power down entire airplane, even disconnect the batteries (to remove power even from hot bus) and reconnect after a minute. (did it with passengers on board once or twice) (eyesroll)