We have a user event script that is triggered before record submit on the Item Receipt. It loads the related Inbound Shipment, sources a value of a field, then copies it to the IR custom field. It works except when the Item Receipt is created from the Receive Inbound Shipment page. The context is bulk processing. Is there some incompatibility between bulk processing and user event script (before record submit)?

did you check the context on the deployment of the user event? That might be one of the ones that is defaulted to not fire

Everything is selected there.