Strange domain controller one way sync problem fortigate s2s vpn

Hi,

We have this system admin team who is complaining that whenever our VPN tunnel is shifted from one internet provider to another the active directory sync will work one way for example from domain controller A to B but not from B to A. Any object created or deleted on B will not reflect on A but any changes on A will reflect on B. Very strange thing as no change except the underlay ISP link.

All traffic flows inside VPN tunnel in both working and non working cases.