It's totally normal, I call it cycle overlap
Your gears maintain momentum and can pull the piston back by themselves, this is especially apparent with larger batteries or heavy prometheus gears. Infact if you get a powerful enough battery, or charge your battery right to its limit, you can get 4 shot burst on semi!
My own M4 gets about 1.3 cycles on a semi shot
The only time I found this to be a nuisance is on the trigger master, it takes that 1.3 cycles on semi and puts that exact current through the motor for semi, but on 3 shot burst it'll put around 3 times that power through so you'll often start out with a 4 or 5 shot burst. Not like it's a big deal you just reduce the burst length

And since we're on the subject, theres also burst overlap, in which each burst is more than 3 cycles (like 3.2), so every 5th burst you'd get a 4 shot burst. Again, also solved by reducing burst length.