15yd facemask foul by K on scrimmage kick.

[Fiji07]

During the kick, before R possessed the ball.

[mbyron]

Then you're right: since it's a foul by K during a loose ball play, the basic spot should have been the previous spot. And if R elected to accept the penalty, the down should have been replayed.

If the kick had ended (by R gaining possession) prior to the facemask foul, then the basic spot would have been the end of the run. In that case, the enforcement you actually used would have been correct.