Wewt.
Long story short, the car has high 11s in it and I wasn't getting out of first and second early enough, but after that pass I called it quits. I habitually put the car in neutral and pump the clutch a few times after each pass as I'm braking, and that time the pedal was super squishy and only returned about halfway for two pumps, a third one and it was fine. No slipping on the pass obviously, and none on the quick pull in the return lane, but it was disconcerting enough to end my day. I've been suspicious of my master cylinder for quite some time now, and this close to solidifies it. The engagement point didn't change all weekend, and I doubt heat would have caused an issue that short-lived or intermittent.
I'm waiting on Scheides' GoPro vids!