Objects render in the wrong order inside guiTSControl

This one is driving me nuts.

Some things are drawing in the wrong order inside my minimap control, like a building draws the roof then the floor and a few steps later it draws it correct.

Also it seems that the water is always drawn above the landscape, so if you have a pond you see a floating water square above the landscape.

I've stepped through as much of the render code as I can imagine matters and I cannot spot the issue.

Can anyone offer advice on how to debug this?