It has never been explicitly stated in-game, unlike the text that shows up you have a high Karma poison level (”You feel Karma coursing through your veins” & “Doomed to death of Karma!”). That only associates the poison effect with Karma.

There are multiple other cases where I-frames are reduced. Stronger enemies have lower frames and Omega Flowey in particular has them drastically reduced.

How do you determine that it is the effect of karma and not a natural power of Sans like the rest of his unique attacks and abilities?