Azure Blueprint Assignment is stuck in "Deploying" state - frozen in an infinite loop

Question

After creating a new blueprint assignment for an existing blueprint, my blueprint assignment is stuck in the "Deploying" state for over an hour (should have taken 4-8 minutes).

At this point I want to cancel the deployment, but I don't know how to stop the endless "Deploying" loop.

Background

Examining the activity log shows that the blueprint assignment is getting stuck provisioning a key vault. The logs show the following three operations repeated many many times with each set reappearing at 3 minute intervals:

1. Update Key Vault - Started
2. 'audit' Policy action. - Succeeded
3. Update Key Vault - Failed

The error reported in the failure is "InternalServerError - Internal Server Error (HTTP Status Code: 500)"

Attempting to unassign in a "Deploying" state

Activity Log showing infinite loop

Attempted Solutions

I've tried clicking "Unassign blueprint" in the DevOps Blueprint page on the offending assignment but get the error: Removing blueprint assignment 'BLUEPRINT-NAME'. The operation is not allowed when assignment is in 'Deploying' state.

I also tried removing it using powershell: Remove-AzBlueprintAssignment -Name $blueprintAssignmentName -SubscriptionId $sub
but received the same error: The operation is not allowed when assignment is in 'Deploying' state.

Is there any way to cancel this assignment while it's deploying?