By default, YayPay is considering the ERP system as a system of records for contacts. So if you've updated a contact in YayPay, but different information arrives from ERP, this contact information will be overwritten.

This is also related to the “Billing Contact” flag - if you set it and then see that it disappears, it means that it is replaced by the value from ERP.

However, there’s an option to use YayPay as a system of record for contacts.
To enable this, select an option Use YayPay as the system of record for contacts on the Settings > Business page. 
Note, that once this option is selected, all the changes made to contacts in ERP will be ignored and the ones in YayPay will be used instead.