Hmm I'm not too sure as well because I did saw a comment talking about how Sans would kill them for going out with Ralsei and Chara, have you tried staying a second time in the bar before moving on?
Sans only kidnaps you if you don't return his affections on the date. You know you're on the good path with him if your first dessert option hanging out with Ralsei is a cinnamon pretzel.
Maybe your affection with Sans is still low or you're missing some options? You might want to save religiously and fuck around and find out at this point :')))
