It is a feature request because syncing an email with an invoice was not available before. The email field is not on the invoice table in Method to sync over. For the email to sync, we most likely will have to add it to the invoice table and add it to the sync code. It's not a bug because nothing technically broke.
To give a little history, this year Method customers were put on the new and improved V3 sync for QBO. Take a look at our article on the QuickBooks' IPP. This was in conjunction with QuickBooks switching everyone from the Classic QBO view to Harmony, which uses the V3 sync. (I think everyone is over now) QuickBooks Online is ever evolving and we have functions that are new in QBO that we have to develop to sync with. This is one of them.
Greg