This is turning into a weird rabbit hole, no offense @akosikoneho. ; )
You can construct a dozen simple example “sampalin” sentences where “I” am the slapper, and the pronoun is always “ko.”
You can construct a dozen simple example “sampalin” sentences where the “you” receiver of the slap is always “ka.”
I’m hard pressed to create ANY other verb-first, simple, non-inverted “sampalin” example sentences that use “ikaw” as the slap receiver, other than the single instance we’re debating...
Why is it “ka” in every other single instance and not this one? There may be some reason, but it doesn’t naturally follow that “ka” should switch to “ikaw” in this case...there has to be some external rule or norm that makes it so. Bituingmaykinang and akosikoneho have given some evidence to that effect, which is interesting.
I’m not really making any argument here other than, that as a method of teaching, it’s a good idea to tell people...
“Don’t use “ko ka” or “ko ikaw” in this kind of structure where you might be tempted to. Use “kita” instead.”
(this goes for the simple cases of course, in more complex sentence structures it’s something else...”Gusto ko na ikaw ang blahblahblah”, for example).