Looking through the forum and bugs category there does not seem to be anything about Custom Fields from the Credit Notes module not being populated in the General Ledger Transactions in Custom Reports. This can be seen in the picture where there are no fields for Credit Notes
Also, in General Ledger Transactions in Custom Reports, the Customer and Billing address fields for Credit Notes do no populate in the records as other Credit Notes fields like Account, Description, Qty etc.
Custom fields in Customer module do populate in Sales Invoice records but do not populate in Credit note records in Custom Reports when General Ledger Transactions is selected. An example of such Custom Field is “Note”