Just to clarify....

There is absolutely no issues with isolating non-fabrication objects (of any kind) with the Maskview command. This command modifies the VISIBLE state which is a boolean true/false state that every object with a visual presence has; It doesn't get any simpler than that.

However, it gets a lot more complicated and mystical when talking about isolating within a given viewport, but only with regards to native Fabrication objects.

If i had to guess, i would bet money the blocks were isolated (had their VISIBLE state set to false) and then the layer they resided on was locked. In order to change an objects VISIBLE property, the object cannot be on a locked layer.