Within the context of the game these are two different things?
'Flash KO' is an early knockout.. before a fighter has received much damage. Not always a 'fluke KO' but an unusually short KO compared to the 'average' amount of a damage a fighter needs to inflict before a KO.
The term 'flash KO' was used in the 2009 iteration to describe the end of a fight by KO before the traditional amount of damage to cause a KO was reached. They were entirely random and fairly rare.
A 'one punch knock out' is exactly what it sounds like and doesn't distinguish between the amount of damage inflicted up to that point. You can have a one punch KO that is functionally a 'flash KO' and also have a 'one punch KO' at the end of a 5 round war in which a fighter was absolutely punished and sustained an incredible amount of damage.. and that wouldn't be a 'flash' KO.
set:subset relationship
Regardless of this distinction in the terms I think the problem most people dislike is the lack of variation in finishes... too many lights-off finishes compared to how real MMA fights end (dropped, scramble, finish on the ground, ref intervenes, etc.)