No flaw, just whenever you restart the process with a combination in tow, replace the word "key" with "combo"

So process for exact example executes as such:
Input P, it is a start of a combo, so allow new input
Input P (PP), it is still all the start of a combo, so allow new input
Input K (PPK), still following combo, so still allow new input
Input P (PPKP), which is not part of a combo anymore, so the PPK that's been stored up gets executed, and awaits for more user input since this new P could be the start of a combo

However, this gets awkward for long combos
Most games don't delay your normal actions until it can figure out exactly what you're doing
Your character will likely punch, punch, kick, and kick, then do their combo, or the last kick is their combo
Then sometimes it's all a matter of how you want it to work as a game
Assuming you have combos KK, KKP, and KKPP, do you really want to be able to string all the combos together? Otherwise you'll have to allow for some sort of pause in there, or a different way to choose which one you're pulling off (or just avoid doing such silly combinations altogether)