Bug? Assigning material to block instances

Let’s say I create a simple block with just one box inside it. I set this box material to be By Parent : I want to be able to set this box material at the block level.

By default, the instanced box indeed changes material according to the layer it is on. But if I try to manually set my block instance material, I get the following dialog:

Bug : the dialog reappears 3 times before my choice is taken into account.

But the real bug to me is when I have multiple block instances selected, and want to set one specific Material to all of these :

Apply to all is supposed to be a checkmark and is instead a button : I cannot choose No to all my block instances at the same time.

The fact that this dialog even exists is bizarre to me, and makes me hesitate to check Don't ask again : in which case is this dialog really needed ?