$push in window aggregate in classic stores the value "MISSING" when it finds a document with missing value. In SBE, $push ignores  missing (i.e. NOTHING) values. As a result, a collection with a single document with missing value sends to the next stage in classic an array with a single element (MISSING) and in SBE an empty array. The implementation of push in classic should be updated to ignore missing values in add and remove methods.

$push accumulator in classic correctly ignores "MISSING" values.