It's possible there is no connectivity between two VMs on different subnets and different computes, as long as the subnets were created after one of the vms was started.

Steps to Reproduce:
1. Create a network, ipv4 subnet and router
2. Start a VM on this subnet
3. Create an additional ipv6 subnet on the same network and create another network and subnet with ipv6 and ipv4
4. Add these 3 new subnets to the router
5. Start a VM on a different compute
6. Try to reach the new vm from the old one on ipv6

Actual results:
There is no connectivity. Although both vms have ipv6 addresses assigned and connectivity on ipv4 works as expected.