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Decals with UV from geometry, not sorting correctly

I’m trying to understand why I can’t get some decals to sort properly. I need to layer up lots of geometry (think: a road with lots of different coloured stipes). The road stripes are geometry which is UV’d (I can’t do them as pure texture as from the driving point of view, the texture mips out too quickly and the lines become soft in the distance]. The area the stripes geometry covers is a few city blocks or more.
For each layer of stripes, I import the geometry with it’s UVs and add a decal material to it. I then place that geometry in a decal actor and set the sort order.
The result is not at all what I’m expecting. Below are some sample chunks of geometry showing the UV mapping and a screenshot from UE. The green should be on top of the red, and the blue should be on top of the green. As you can see, the sort order isn’t respected. What’s even weirder is that if I save the project and come back in, then sometimes the green layer doesn’t sort properly.
So how do I do what I want to do? I need km’s of stripes, all UV’d in the modelling tool, to coplanar sort using decals while respecting the modelled UVs.