Cannot unlink materials

[tfboy]

Hi,
Using KS 6.1 Pro.
I have an issue which I cannot determine whether a feature I've missed or a bug.
I have a model with a row of monitors that I initially linked together and used the box map feature to apply a label (screenshot) to the monitors. I have then inidividually unlinked them where necessary in order to provide a different image.

On all the materials apart from the problematic monitor, I can see the material listed in its properties - 001 png attached.

However, on one set of screens (the 55inchtv), I cannot unlink for some reason. The material's properties doesn't show up. Clicking on the Unlink Materials button doesn't do anything. - 002 png attached.

Any idea what's going on?

[bdesign]

Can you unlink if you right-click the object in the scene tree>Material>Unlink Material, or right-click the object in the viewport>Unlink Material?

[tfboy]

Can you unlink if you right-click the object in the scene tree>Material>Unlink Material, or right-click the object in the viewport>Unlink Material?

Nope, doesn't work.

However, as a workaround, I can apply a new material by dragging / dropping onto the Parts tree.