That's not what he's been doing down the right, he's been too safe if anything. I haven't checked the stats but I'm fairly certain his successful take ons and goal involvement ratios, per match are much lower than they are at City down the left. I think they'll be lower than Saka's been showing down the right for England when he has played too, which is why I don't think it's about the instructions he's been getting.
I'd prefer we at least try him down the left, where he actually has the experience at senior level. I suppose he's building that experience now with England but my point was his place isn't as secured in this team like Kane's or Sterling's. He might be more comfortable down the left(especially when coming on as a sub) which would help him get into the game faster and find the form we're used to seeing from him.
I have noticed, at least one of the times Grealish has come on, Southgate moved Sterling over to the right for him to have the left side. So I don't see where the reluctance is coming from, other than Sterling preferring that side. Maybe it's Foden saying he wants to play down the right because that spot is kind of open for someone to make their own.
I'm normally biased towards City players but I couldn't argue too much if Saka starts ahead of him down the right, based on how they've both done so far in the minutes they've played. Sakas been good to his credit, Foden is capable of more than he's shown.