Maybe this image will clarify what I mean:
The bucket has the object type: World Dynamic
The jerrycan has the object type: World Dynamic
An invisible mesh at the jerrycan nozzle has the object type: Destructible (which is not even relevant, it just has to be another object type)
The water particles you see are set to produce collision events ONLY with Destructible and Projectile.
I’m counting the water particles that hit the nozzle, basically.
The thing is, right after starting the unreal editor these water particles produce collision events for everything they hit (the bucket, the jerrycan, etc), thus, making it impossible to acutally track the particles that only hit the jerrycan nozzle.
Now, in order to fix this behaviour, I have to set one of the Collision Types (which at that time are: Destructible and Projectile) for the particle effect to something else, like World Dynamic, and change it back to Destructible/Projectile. Basically, I do not change anything, I select something else and reselet the initial value.
This fixes for some odd reason the problem and after that the particles behave as expected (only producing collision events while hitting the nozzle).