If anyone is interested in how to workaround this problem:
I have an inventory system with a HUD widget that renders output from a Scene Capture Component 2D. This component is on a blueprint spawned off the character far outside the skybox. What I do now is disable ticking of the capture component on beginplay, and only re-enable ticking of it when the inventory ‘inspection’ window is activated on the HUD.
This means GPU Particles now collide correctly during gameplay.