We noticed some customer invoices on registering payment, it shows message "Outstanding Payment Available" which the same payment we registered for this invoice. also the "add" payment link shown under the invoice amount is not working.

more investigation shows the payment matching is done on the database level on table "account_invoice_payment_rel" , but no matching number ( e.g. like A5212) has been assigned on the GL entries on the payment , and thats probably why is showing this message 

our question is what is the reason behind this sudden behavior and how to fix it ?!

Odoo 11.0-20180103 (Community Edition)