If you use collision, every time a single particle hits an object (assuming you have collisions enabled) it will then go through the burst mode. When you create a sub-emitter it’s just creating another particle system as a child and telling it to emit based off the collision. By default this particle system will be set to looping at the very top in the main particle system module (second setting from the top). I’ll note that even if I had looping enabled with the sub-emitter, mine didn’t loop after the initial burst. So disabling this didn’t change anything for me, but it could help for you.
If you just want it to burst once upon impact, uncheck “looping” at the very top and ensure the burst settings just say 1 for the interval. I’m not sure why yours was not setup properly initially. I didn’t experience that issue when testing it out just now.
