Inexplicable Landscape WPO issue?

I’ve been trying to get this pretty simple WPO on my landscape material to work:

The ‘slope’ function is very simple:

This works fine for the ‘base colour’ part of the material output, but for some reason the WPO simply will not function - if I remove the slope-based lerp then it performs the WPO (though to a far larger degree than it should, more like 100m than 100cm), but the lerp just refuses to work (rerouting the WPO logic inc. the lerp to the base colour functions perfectly however).
I’ve tried moving the slope logic out of the function and directly into the material, but that also did not fix the issue.

I have no idea what’s going on and I’d love some help.