It is possible to get wrong results if we do not fold the arguments of a valueBlockLogicalAnd when the input data has arrays.

Consider for example the arrays A1 : [true, false] and A2 : [false, true] . The arrays are for the same document but another predicate has been evaluated for each one.

If we first run AND and then FOLD we get  [false, false] -> false, i.e. the document does not qualify

If we first FOLD and then run AND we get true AND true -> true , i.e. the document does qualify

Additionally, all methods that take a bitmap as an argument should fold all other inputs to make them comparable.

Example: {$match:{ "a.x" : {$lt: 10} } }, {$match: {$expr: {$map: { "input": "$a", "as": "arr", "in": {$add: ["$$arr.x", 10]} } } } }

{$match:{ "a.x" : {$lt: 10} } } will return a a bitmap on the measures.

{$match: {$expr: {$map: { "input": "$a", "as": "arr", "in": {$add: ["$$arr.x", 10]} } } } } if we don't fold it will use a bitmap on the measures and a block on the elements.