Update: had another match where I was against a Witch-king, but when the he was summoned I already had Anduril out and a couple of spirits on the field. I equipped Anduril to one of the spirits and didn't encounter the bug the same as last time.
Although another variable that changed is that this time, my opponent already had already tempted the ring once before casting the Witch-king, so I'm not sure which one is the issue yet.
Update: had another match where I was against a Witch-king, but when the he was summoned I already had Anduril out and a couple of spirits on the field. I equipped Anduril to one of the spirits and didn't encounter the bug the same as last time.
Although another variable that changed is that this time, my opponent already had already tempted the ring once before casting the Witch-king, so I'm not sure which one is the issue yet.