Trouble with Layers and Z order

As shown in the image, I have three layers, "backbottom", "backmid" and "main". They all have Z elevation 0 in the properties.

My problem: In the Z order bar, however, "backbottom" appears on top of "backmid". I can't seem to reorder the layers, only objects.

(all objects on "backmid" layer have higher Z positions than all objects on "backbottom" layer.

Perhaps related to this is another weird behaviour: objects with negative Z position disappears in the editor (but are visible at runtime).

The order of layers is done in the layers bar to the right. The z order bar can used to change the hierarchy of objects within the given layer. I don't think the order of layers in the z order matters or has any effect, not sure you mentioned a problem? Are the objects doing something you don't expect?