I set the ARM nodes back to v2, but also ran into an issue: I thought that by having a volume size set in the default settings, it would be inherited by all nodes. But this assumption was incorrect, and while the v3 nodes work without the volume specification, the v2s are failing to boot. I'm reopening to fix the v2s, and then this will be complete.
ARM hosts aren't booting with v3 flavors in sandbox testing. Opened ticket VLX-264018 with Vexx to address this issue.