Sans's reaction is immediate. It's convenient that Frisk was just grabbing hold of his sleeve, because it makes it far easier for him to sidestep the two of them right out of there. He goes from giving Frisk a helpless look ("what do you expect me to do?") to disappearing in a matter of moments.

The mud crashes down, and it's only when Sans and Frisk emerge from the other side of the shortcut that he realizes it was less of an attack and more of a distraction. He sees her fly off into the distance, and his desire to chase her is extremely minimal.

This is bad. Someone got to Lapis, and bad.

Sans looks down at the mud caked onto his clothes and entire body, rubbing some of his off his coat, and then prying a finger into his ear hole to try to scoop it out.

"Well, that was a wash," he says. "...In that it left me needing one."