Payment method blanked out for Miscellaneous Receipts

We are in the process of setting up new Receipt sources, to assist with a process improvement project underway in our Accounts Receivables team. All receipts are entered in batches, with separate batches for Cheque and for EFT receipts, but both currently using the same batch source.

When using the old Receipt Batch Source, if the Receipt type is changed from Standard to Miscellaneous, the Payment Method remains, which is good. However with the new Receipt Batch Sources we have defined, when the Receipt Type is changed to Miscellaneous, the Payment Method is cleared by the system, and needs to be re-entered. I have looked at the way the Receipt Batch Sources and the Payment Classes / Methods are defined, and cannot see any obvious differences that could cause this to happen.