Unless I've missed a setting somewhere, it appears to me that sub-item sorting goes on in a grid whether the function is engaged or not. I cannot demonstrate that the grid menu item "sort criteria applies to sub-items" is not engaged so you'll have to take my word for it.
The image below is from my contact list
filtered for "S"
sort item asc
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and after all, subs don't really need to meet the source to be in a grid... right ?I'll stop there to keep it simple).