The following pipeline returns different results with optimisations disabled via the "disablePipelineOptimization" failpoint:

// Collection contains only the document: {outer: [{inner: 0}]}
[
    {
        $project: {
            flattened: {
                $map: {input: '$outer', as: "iter", in : "$$iter.inner"},
            },
        },
    },
    {
        $match: {flattened: {$elemMatch: {$eq: 0}},
    }
]; 

• With the optimisation it returns []
• Without the optimisation it returns [{flattened: [0]}]

The rewrite turns the match expression into {$match: {"outer.inner": {$elemMatch: {$eq: 0}}}} which is not exactly the same in this context due to implicit dotted array traversal. [Mongo docs.