Pretty sure this is related to a fix/change in 15 that updated how timers/repeat counts/etc tracking in effects was handled. I see it too in that about once a minute, the effect running seems to “fade out” and then restart (one of the fairy light effects). Under 14.x it worked continuously.
From what I’m reading, a change in the effects code broke a few effects and it seems that is being fixed for 15.1.
You can read about a similar report (and think same problem) here:
So for now, prolly either 1) live with it til 15.1 is released, 2) revert to 14.x or look for an try one of the 15.1 beta/nightly builds