GS768T: Untagged management vlan

Model: GS768T v5. Firmware 6.3.1.19.

768 port1 goes to a port on a GS116. Nothing else connected to the 768. Management PC connected to GS116, as well as DHCP server.

Trying to get the 768 configured ahead of time before rolling out out elsewhere.

I have created and assigned an new untagged vlan ID to port1 on the 768. As soon as I tell the mgmt to operate on that new vlan, I am unable to access the mgmt interface anymore and have to do a factory reset to get the mgmt responsive again. Whether recieving DHCP or static mgmt IP, the issue is the same.

A single vlan on an untagged port should be viewable by a non-managed switch connected to that port.

Is it possible the mgmt vlan on a GS768T needs more than a simple untagged vlan?