in the clip below, if you look very closely, you can see that every time the particle happens, the spritesheet of the particle animation is visible for a split second (most of the time)
https://gyazo.com/39367c6d7dbd057c5b340b47185eb727
the spritesheet just appears out of nowhere for a split second. it seems as if the particle flipbook system actually has a tiny delay before it starts working, hence the spritesheet being visible for a moment. Other than that i have absolutely no idea why this bug happens
Edit: As a side note, i use particleEmitter:Emit(1) method to make the effect appear.