Since Apstra 4.1.2, when using the Junos platform for ESI racks, the user must enable a virtual gateway on Layer 3 virtual networks. This is the requirement with the current reference-design. 

What is the reason behind?

Virtual Gateway requirement for an IPv4-enabled VN was deemed as a reference-design requirement, as protocol-specific (BGP) and other protocols will have issues when ingress traffic lands on the wrong borderleaf/leaf for an ESI pair.

This requirement has been implemented in Apstra 4.1 (AOS-25311) by introducing a build error. No UI implementation (e.g., automatically select the virutal GW for IPv4-enabled VN and gray it out in UI) has been done as of Apstra 6.1.