Artifact cannot burnish 5th socket

I have two Red-level artifacts. Galaxy Ring has 5 sockets open and in use and burnish/aura is working correctly.

For Dark Knell, 5 sockets are open and in use, but burnish/aura doesn't work for fifth socket.

When I try to burnish, I get a message that says "All slots must reach the same Aura level to be burnished."

If you look here, it doesn't even seem to be registering as part of the artifact. If I take the glorystones out, Galaxy Ring shows 5 available sockets. But Dark Knell only shows 4.

Under "Current Aura", Dark Knell shows just the 4 empty circles when there should be 5 empty circles the same as Galaxy Ring.

I have a feeling this is going to require an actual support ticket, but if anyone can tell me if I'm missing something I would really appreciate it!