Currently in Classic, $bottom accumulator will compare arrays element-by-element, which is natural, but contradicts our documentation: https://www.mongodb.com/docs/manual/reference/bson-type-comparison-order/#arrays

A less-than comparison, or an ascending sort, compares the smallest elements of the array according to the BSON type sort order.

A greater-than comparison, or a descending sort, compares the largest elements of the array according to the reverse BSON type sort order. 

SBE will follow the docs.

This is because in Classic, accumulator wraps sort keys in array and sort key generator considers the array being nested, so it returns it fully in a sort key. SBE accepts document fully as-is, so the array is not nested, so only one element is returned as a sort key.

Reproduction script is attached.