SSE with CMK & ADE

So today I was trying to attach a data disk to my VM and I get an error due to mixed encryption types. All of my disks are SSE with PMK & ADK apart from a lone disk which is set to SSE with CMK & ADE.

According to the documentation, this doesn't seem possible, and the errors in the UI hate it (cannot have both Azure Disk Encryption and Encryption at rest with customer managed key enabled. It is currently encrypted with 'Azure Disk Encryption').

From the docs, this isn't something I can change, but I cannot attach a new disk to move data to as the disk causes loading any changes to the availability set to fail (since one or more disks in the Availability Set have validation errors..).

If anyone can offer some guidance I'd really appreciate it!