He did show signs last season that he could play as a wingback, but the current implementation is a bit different imo. I can't remember who played RWB when Sane was LWB last season but it was probably Sagna or Zabaleta, and they usually played pretty defensively, whereas Walker fully subsumed the RWB role and attacked as a winger many times, which saw Sane defending more often than not.
Ultimately it comes down to a partial fault of Sane and Pep on this, is Pep instructing Sane is defend more or does Sane think he needs to be defending alot more than he normally did last season. Also, the season is very early and I think Sane starts off slow in general and he will pick it up.
On to tactics, Everton predictably went 5-3-2 and it was obvious, Pep probably knew that, he should have went with a formation to neutralize that like 4-3-3 where he could overload the wing space. This was evident when Bernardo linked up with Danilo fairly well for the small time they had getting behind Everton.