SERVER-113172 implemented functionality to split MatchExpressions into single table residual and equijoin predicates. This function fails if the expression references a variable that is not in the let statement of the $lookup and is not a field path from the local collection.

This means that expressions like {$expr: {$let: {vars:

, in: {$eq: ['$a', '$$b']}}}} are considered not splittable and thus we bail out of join optimization, even though the expression is a single table predicate and can be pushed down into the find layer.