For compound index, when the first field is used in $lookup, SBE only generates index key with a single field instead of all fields

{$lookup: {from: coll.getName(), localField: 'a', foreignField: 'a', as: 'array'} 

The generated keys are {a: 1}, {a: 1} instead of {a: 1, b: MinKey}, {a: 1, b: MaxKey}

https://github.com/10gen/mongo/blob/b5f58d8cc3853c3bdecba44bf3fdf419fa200cf8/src/mongo/db/query/sbe_stage_builder_lookup.cpp#L885