I see what you mean. It looks like the culprit is the Scale Over Life behavior. [...and this has to be a bug!] It seems to only take effect when the Set Speed retiming behavior is applied. [So... *it's not you!* -- go to Apple Menu > Provide Motion Feedback and leave them a note about this.]
To fix, try this:
In Scale Over Life, select Increment Type: Custom.
Turn down the disclosure triangle on Custom Scale. If there isn't a line with 2 end points (and there's not likely to be), delete the behavior and add it again and set it to Custom Scale (the line with end points should be there).
Click on the first (left) control point and on the custom scale slider below, double click on the number and set it to 100% (if it's not already that.) Click on the right control point and double click on the number for the custom scale slider and type in 0. (Trying to drag those things with a mouse is more trouble than it's worth!)
That *should* get rid of the sparkles. If not, increase the cell Life slightly (about 0.65 or so) and drag the black opacity swatch on the Color Over Life gradient to about 70 - 75% (you still might get a slight "flash" of sparkles on the last frame -- but I'm fairly sure just dealing with the Scale Over Life behavior will fix it.)