Your on-premises data center has two routers connected to Google Cloud via separate VPNs, but all traffic flows through a single VPN instead of load-balancing across both connections as intended.

During troubleshooting, you observe the following:

• Both on-premises routers use the same ASN.
• Both on-premises routers have identical routes and priorities.
• Both on-premises routers connect via VPN to a single Cloud Router.
• VPN logs contain no-proposal-chosen errors during connection attempts.
• The BGP session fails to establish between one on-premises router and the Cloud Router.

What is the most likely cause of this issue?