You’ve accidentally RESET the timeline, You RESPAWN in the ruins of the underground as Sans was dealing with his own problems: He knew when RESETs happened and thought you used this as a way to end your marriage.

You reach Snowdin and run towards Grillby’s, before you can walk in Sans opens the door, making eye contact with you as his face drops for a second before returning to his usual smile

“Whoa careful, pal, ya’ almost clobbered me” Sans side stepped out of your way and disappeared