I have this weird rendering issue... depth seems to be broken?

I’ve been working on this project for about 30 minutes. when i dragged in a navmesh and scaled it up, my engine froze for a couple moments. nothing unusual, BUT when it unfroze everything looked like this:

I don't know how this could happen.

some info on the project:
I use: - Chameleon PostProcess
- A single material which i create instances with my different textures.
- a skylight for lighting

The BaseMaterial:

Something that i’v noticed, is that objects with, for example the default material, don’t appear to render completely through walls. they only get the material of the wall that is infront of them applied, IF they are in one of those areas where the wall is half-transparent (image bellow)

This only appears to occur on materials that have any kind, except masked, transparency turned on in their nodes.
I changed the transparency mode to masked and everything is rendering just fine again.
STILL i would like to know how this occurs. i’ve had this in multiple projects. even ones where i didn’t use any transparency…