OK, I was misled by the errors in the dmesg log:

[   14.186318] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector
ObjectID from Adapter Service for connector index:1! type 0 expected 3
[   14.186458] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector
ObjectID from Adapter Service for connector index:3! type 0 expected 3
[   14.186592] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector
ObjectID from Adapter Service for connector index:4! type 0 expected 3
[   14.186725] [drm:construct [amdgpu]] *ERROR* construct: Invalid Connector
ObjectID from Adapter Service for connector index:5! type 0 expected 3

Apologies for the misconception.

graaskaeg