In this case could one not put Bientot at the end of the sentence?
e.g. tu sors de la salle de bain bientot/a bientot?
Yes, you can put "bientôt" at the end of the sentence. But "a bientôt" is not right at all.
Why is "Tu pars bientôt de la salle de bain" marked wrong?
because pars means leaving, not getting out of! Small difference but there is diff!
Tu sors de la salle de bain bientôt ? Accepted :)