Does this problem apear on particular camera angels?
I’m quite new to the unreal engine (and game development in generall) but I had this problem a little while ago and for me the problem was fixed when I did go to my particle system and pushed the button “set fixed bounds”
I’m not sure of this will fix your problem but you maybe could try this! 
PS: disclamer: I’m don’t know what “set fixed bounds” actually does! I only know that for me that it did fix this problem for me, so just to be sure: you maybe want to make a back up of your game folder before trying this!