SERVER-95751 seems like a good candidate for a PBT. (The fuzzer grammar for $map seems too limited to produce the necessary conditions to reproduce this bug. We want to be able to generate a $map with a field as the 'input' so that the 'in' expression can reference sub-fields, and that a subsequent $match references that field) I was discussing with matt.boros@mongodb.com and there will need to be some work done to support $map and doubly nested arrays.