While pinging to WAN IP from LAN side host, the packet is taking wrong path at ACX-Cisco interop.

Customer was running VRRP at ACX & HSRP at Cisco.

HSRP (Hot Standby Router Protocol) and VRRP (Virtual Router Redundancy Protocol) cannot directly work together because they are separate, incompatible protocols, even though they serve a similar function of providing router redundancy; HSRP is a Cisco proprietary protocol, while VRRP is an open standard, meaning they use different configuration methods and communication mechanisms.

We cannot configure a single router group to use both HSRP and VRRP simultaneously. 

If you need redundancy across different vendor devices, VRRP is the preferred option due to its open standard nature.

Customer changed the config to VRRP at Cisco & corrected interface cabling, post which issue got resolved.