This performance was not a one-off, it a symptom of a structural problem Barca have. Firstly Cancelo is more comfortable on the left than on the right at this point, he became a fullback playmaker when Pep moved him to LB, he can cut inside and go outside more comfortably there than on the right wing.
Secondly and more importantly, this is just about balance. If the second fullback is more defensive, like Araujo was today, Cancelo is free to do his thing with better defensive structure behind him (not that it was good today).