No, changing the MmbId field in filler doesn't change the same named field under CdtrAgt. I could only tell this by saving the form and checking the xml because, if you look at the template, you'll see the MmbId field under the CdtrAgt tag isn't even on the form. I think there must be something strange going wrong during the default setting and there seems to be 3 fields inadvertently set as follows:
Field: /ns1:Document/ns1:CstmrCdtTrfInitn/ns1:PmtInf/ns1:DbtrAgt/ns1:FinInstnId/ns1:ClrSysMmbId/ns1:ClrSysId/ns1:Cd
Default value: xdXDocument:GetDOM("Payments_Current")/dfs:myFields/dfs:dataFields/d:Payments_Current[count(current()/../preceding::ns1:PmtInf) + 1]/@DbtrAgtCd
Set in error: /ns1:Document/ns1:CstmrCdtTrfInitn/ns1:PmtInf/ns1:CdtTrfTxInf/ns1:CdtrAgt/ns1:FinInstnId/ns1:ClrSysMmbId/ns1:ClrSysId/ns1:Cd
Field: /ns1:Document/ns1:CstmrCdtTrfInitn/ns1:PmtInf/ns1:DbtrAgt/ns1:FinInstnId/ns1:ClrSysMmbId/ns1:MmbId
Default value: xdXDocument:GetDOM("Payments_Current")/dfs:myFields/dfs:dataFields/d:Payments_Current[count(current()/../preceding::ns1:PmtInf) + 1]/@DbtrAgtMmbId
Set in error: /ns1:Document/ns1:CstmrCdtTrfInitn/ns1:PmtInf/ns1:CdtTrfTxInf/ns1:CdtrAgt/ns1:FinInstnId/ns1:ClrSysMmbId/ns1:MmbId
Field: /ns1:Document/ns1:CstmrCdtTrfInitn/ns1:PmtInf/ns1:CdtTrfTxInf/ns1:CdtrAcct/ns1:Id/ns1:Othr/ns1:Id
Default value: xdXDocument:GetDOM("Payments_Current")/dfs:myFields/dfs:dataFields/d:Payments_Current[count(current()/../preceding::ns1:CdtTrfTxInf) + 1]/@CdtrAcctId
Set in error: /ns1:Document/ns1:CstmrCdtTrfInitn/ns1:PmtInf/ns1:CdtTrfTxInf/ns1:CdtrAgt/ns1:FinInstnId/ns1:Othr/ns1:Id
Note that I have 7 instances of repeating group CdtTrfTxInf in the form. The resulting xml is a little more complicated in that the first field set in error above, Cd, only occurs for the first instance of CdtTrfTxInf and the last one above, Id, does NOT occur for the first instance CdtTrfTxInf. The 2nd field above, MmbId, is always set in error. The problem always occurs under the CdtrAgt tag.
I can't send you my access table because that's private but if you like I can send you a sample database?