I don't think Jack's role in the treble season was less important and he wasn't playing midfield..
If we carefully look at our tactics in that season, it was something like this:
Bernie-----John--------Rodri---------Grealish
-------------KDB---------Gundo------------------
-----------------Haaland--------------------------
and it was working because KDB and Gundo were old enough to run box-to-box and help during build-ups, so their main job was to create chances and help us score more goals, which they did fantastically. So they were doing the job of wingers in terms of attack and Bernie/Grealish were more involved into build-ups, defence, etc.
Bernie and Grealish were playing behind Gundo and KDB... So I don't think Jack's talent was waisted there, Pep found the best solution for Jack's and Bernardo's skills and we were flying.
Now if we are talking about this tactics:
-------------Akanji-----------Kovacic------------------
-------------Foden-----------Grealish------------------
Savio/Bobb-------Haaland----------Doku/matheus
Here it's obvious that we have classic wingers. Their job isn't build-ups, but to receive the ball and create chances, score goals, make assists.
So for Grealish this is the best position, as like in the treble season, he is behind the wingers, has ability to help during build-ups and has more space.
But difference is that, playing wide we had well-covered his inability to defend well and also I trust him more to receive the ball wide and cut inside, rather than receive the ball in the middle and do something. What I mean is, we don't know how efficient he can be with the ball when he receives it inside, where he will be pressed more, kicked more, etc.
But anyway, what's for sure, he should be always played behind the 2-3 attackers, so he could help us and deliver those preassists or whatever the fuck they are called...