The field REGUP-BVTYP is only filled if the field was filled in the paid item (see BSEG-BVTYP, visible e.g. in the item list or in the document display transactions, FB03).
This field, REGUP-BVTYP, is only used for the partner bank determination during the payment run. Hence, there is no other purpose of this field in subsequent process steps. This is the system standard behavior.
The system only transfers the bank details ID if the indicator T042Z-XBKKT (bank details are required entry in master record) or
T042Z-XPGIR (indicator that the payment method is the post office bank current account method or postal check current account method) are set in Customizing of the payment program (FBZP transaction) when you enter the payment methods or country.
Additionally with the modification notes 524942 and 390740 (not contained in the standard), it is also possible to get Partner Bank
Type filled in.