If we have three documents in a single bucket,

{a: 1, b: 1} {a: 2, b: 0} {a: 1, b: 3}

and a pipeline with a $match for a: 1
followed by a $addFields: that adds 5 to b
followed by a $match for b < 7

The result should contain just the

{a: 1, b: 1}

document because the second has been excluded by the first $match, and the third one by the second $match (3+5 is not less than 7)
Right now I think the first mask is used inside the second $match, but the comparison operator ignores it because it's faster to just process all the data instead of checking the bitmask one by one.

The SlotBasedStageBuilder::buildVectorizedFilterExpr should take care of creating an AND between the existing bitmask and the result that is going to overwrite the kBlockSelectivityBitmap slot in order to preserve it. Maybe with a coerceToBool on its arguments to ensure that the bitmask is always made by booleans