I noticed this too in the market last year, the guy returned my gf's khawp khun kaa with the same
When I asked why he said that instead of khrap, and was he a LB, my missus said that it is fine to return with the same.
I did try to ask her more about it, but by the time I caught up with her she had her head in a bag of locusts!! so I never got any further
