I've experience this too, not that frequently, and I've heard a couple of different theories. One of which says that it's supposed to happen with really quick and high rpm shifts to save on clutch travel when it comes to the following shift. I partially believe this because when it does stay depressed at about 80% the clutch is still engaged and it is not slipping. Another theory is that the factory clutch line and flywheel bolts are subpar. With the line not being able to handle the hydraulic pressure and the flywheel bolts not being fully torqued and slightly loose resulting in a flywheel that can flex at high rpm shifts. I agree it is kind of annoying and your mind messes with you when the pedal doesn't fully return but like I said before, when it has happened to me, the car does not act like its still depressed. Someone correct me if I heard wrong or know any other issues.
Best 1/4: 1.90 60ft. - 12.15 @ 119mph on Street Tires.
460.25rwhp & 408.28rwtq SAE on a Dynojet 224xLC
Tuned by Matt Lepkowski, owner and operator a Performance Evolution in Smyrna, DE.