GPUParticles2D always local, even when "Local Coords" is disabled

[gritty grove]

I'm having an issue with Local Coords in GPUParticles2D. If I try to use a custom particle shader, the particles are always in local space, even if Local Coords is turned off. This persists if I then change the material to a standard Particle Process Material. However, if I start with a new GPUParticles2D and then make a Particles Process Material, Local Coords works as expected. Until I switch to my custom shader. Not sure if I'm doing something wrong or it's bugged.

This is the shader I'm testing
https://pastebin.com/xm6JmPG5

The colored emitter is using a ParticleProcessMaterial and works as expected

The black emitter is using my custom material and the position of the particles is always local (even though the rotation/scale isn't)