Page Background Layer - XX5 Bug?

[Ampa]

How odd! - today I can click the Lock on Background layers, and it works just fine!

UPDATE: Figured it out!!

I have 'Current layer is always visible and editable' turned on in the Options. Page Background can't be unlocked via the Lock icon when it is the current layer. Move to a different layer, and the Lock icon is suddenly active. This seems like odd behaviour to me!