PayPal Issue - No transaction showing in Magento

[SJ_Med]

Running 2.3.5p1

Critical issue - when a customer tries to run a transaction, we first see a $0 transaction in PayPal on our PayPal Manager, then 8 seconds later, the customer sees a rejection with no error message in the error line, however PayPal records the transaction, with a transaction ID from our system, we get a transaction in the transaction log, but no transaction in our Magento system.

This has been going on for several days now and can't seem to figure anything out. 

If the transaction log an PayPal Manager have the transaction, why isn't the Magento system recording it as a sales/transaction, and showing the user that their sale went through? And if we're getting their money authorization in the backend, but no transaction, we can't effectively process the sale, because we don't know what they bought and it isn't recorded in the system anywhere.

Help!!!

[SJ_Med · Accepted solution]

After a whole bunch of site rebuilding, and some really great sleuthing by Nexcess, they discovered that there's a long-standing bug in Magento that I've stumbled across, across the cause isn't clear. I'm posting here to raise awareness in case it affects anyone else viewing this thread.

2.2.0 Checkout - Unable to save address. Validate class not found from basename 'Email Address #10821 - Since 2017 - v 2.1.9 through at least 2.3.5p1

https://github.com/magento/magento2/issues/10821