If the block originally was drawn on layer 0, it will be inserted on the active layer. If it was drawn on one or several other layers, these layers will be imported if not already existing, and the various elements will be on those layers.
The insertion information is saved on the insertion/active layer, even if the block elements are lying on others. If you switch the insertion layer off, the block elements on those other layers disappear too.
So it shouldn't be what you describe. I suspect you have checked the layer status of your block with "Edit the active block in a separate window". With that window the original block layer is shown, probably 0, as this is preferable for a block drawing.