Sorting by a subtotal column sorts incorrectly

[kgaspard]

Repro'ed on customer instance (link hidden for privacy, in Slack with @ContrastingSounds )

From @ContrastingSounds : "OK root cause found – the sort process doesn't yet taken into account when the user chooses to only show a certain level of subtotal. It is sorted correctly - but only if the user takes into account some data they can't see! When the user chooses only to show a given level, it should sort by the visible grouping not by all of them."

So, if I had chosen the leftmost column as the subgrouping level, it would have sorted correctly.

Note this issue happens in the repo branch with subtotal sorting