shouldn't we also keep fetching them at once for expanded choice fields as well ?

Forget that. The loop above for expanded fields does not call the choice list in that loop.

Is there a way to write a test ensuring that a submission of a multiple choice type calls getChoicesForValues only once during the processing, to prevent similar regressions ?

Thank you @xabbuh.

For record, denied as bug fix for 4.4 #48208, accepted as feature and already merged in 6.3 #48342