This Blueprint was working perfectly on 4.18 but doesn’t on 4.21 . Is that a bug?
It seems specific to Mesh Particles. The Material is created and updated properly, the node “Create Dynamic Material Instance” just doesn’t hook the material to the spawned emitter anymore. “Set Material” doesn’t work either.
Any help would be great. 
[EDIT] Apparently it is a bug…bummer. Unreal Engine Issues and Bug Tracker (UE-59923)
Somebody tried to hack the overrides but it doesn’t seem working on 4.21. At least on my project.