A mi me ocurre lo mismo, llega a copiar el sgtxt de la bseg original en las entradas de la regup pero luego el documento ZP creado ya no las tiene.
Una solución que se me ocurre es hacer una sustitución FI a nivel de documento completo para que lea de la regup o del documento original y le copie el campo sgtxt.
En esto coincide nuestro amigo Soumya Rao en scn:
The field BSEG-SGTXT is not filled into the payment/clearing document by F110 because a payment document can be used to pay a lot of invoices and in this situation the payment program will not know from which of the many invoices the BSEG-SGTXT should be taken from.
If you want to have SGTXT from the invoice in the payment transfer medium you have to write a “*” in front of the SGTXT from the invoice. In this case the value from SGTXT is taken into REGUP-SGTXT and can be used for the payment medium.
However, the same text will not appear in the line item text of the payment document. If you want to see the same text in the payment document:
One solution would be to create a substitution. You can create a substitution in accounting documents at callup point 2 to fill SGTXT with the vendor code LIFNR for example.
If you want to have the SGTXT copied from the invoice you could create a clearing rule for SGTXT.
Also you can make use of BTE 00001120. You can find documentation for the usage of this event in transaction BERP, enter process interface 00001120 > execute > Keep cursor on the event 00001120 and then click documentation.