It's my understanding from MS support that the Comment field and the Transaction Information fields within the General Journal window don't have a destination column within the posted GJ Entries table. This idea is to add those two fields to the posted table so a custom extension doesn't need to be created.

Message from MS support regarding this issue:
Since there isn’t a destination column for this field to go to the data is not transferred. It is similar on how the COMMENT field functions on the line. Once posted the COMMENT field is gone since there is no historical/archive column for that field. You could create an extension to have that field retain the value while it moves to history. So I think the extension would need to be on the Gen Ledger Entries table to add a custom field and populate it based on what is in the Transaction information column. That is what I had another Partner do with the Comment field.