If the opacity of a block (call it block A) is modified using the Multiblock Inspector, and then another block (call it block B) is hidden by unchecking its box in the Multiblock Inspector, block A will also become hidden, even though its checkbox remains checked.
If block A has its opacity removed, by Control + click on the icon, then things return to normal and block B can be hidden without hiding block A.
This seems weird. Is it a bug? I’m using v10.1 on Windows 10.