Hello Sara,
that is strange. The logic is the same for sales document and billing document.
If you have debugging capacity, the relevant source codes are the following.
Sales order
FV45PF0P_PREISFINDUNG_VORBEREI
...
IF vbak-vbtyp NA 'W'.
... here system fills TKOMK-TXJCD
Billing document
LV60AA58
...
data: jcdactive.
... here system fills TKOMK-TXJCD
Best regards,
Ivano.