This might occur on any Bosh managed deployment built on vCenter.
In certain scenarios, the VM-VM Affinity rules created by Bosh may be removed from vCenter. If these rules are removed, Bosh will not automatically add them back.
This problem can be resolved by manually creating a VM-VM Affinity rule with the name of the AZ, the type of rule, and _vm_group_rule at the end. Example for a MUST rule that matches Bosh named Availability Zone "US-TKG-AZ1":
US-TKG-AZ1MUST_vm_group_rule
If this is a SHOULD rule in Bosh, replace MUST with SHOULD.