You cannot set Stored fields in

beforeLoad

when editing an existing record, only when creating. From SuiteFlow Help page "Before Record Load Trigger":

You cannot set field values on an existing record if the Set Field Value actions execute on a Before Record Load trigger. Use the Set Field Value action with a Before Record Load trigger for new records or for setting workflow fields on new or existing records. Use a custom action to update fields on an existing record.

Same applies to Scripts.