Robbie!
Great challenge...it only happens when warm, affects all four wheels and you have had a experienced mechanic to check the complete system...then we have to turn into some wild guesses:
Rubber brake hoses could be damaged on the inside (and look like new on the outside) and cause some strange sensations in the system (but your car is a little bit to new for that).
If it affects all four wheels - master cylinder?
Crazy thought...check the pedals and linkage so no electric wires, hoses, mats or other interior is interfering with the pedal return.
Hope you solve it, good luck with your detective work
