CustomValidator strangeness (v4.2.3)

[ed-ud]

My CustomValidator validate() method that I've attached for dcim.interface is seeing, in the "instance" parameter, the list of tagged_vlans that are pre-change.

When I edit the Interface and remove the existing VLAN from tagged_vlans and add 2 new VLANs, the validator sees only the original
VLAN using instance.tagged_vlans.all() (and not the 2 new VLANs that I added).

Anyone else seeing this kind of thing?

[ed-ud]

When I try to remove the one VLAN from the tagged_vlans list, the validator sees that one VLAN (even though the change should show NO VLANs because the tagged_vlan list is now empty).