[{arrayName:[{fieldName1:{fieldName2:"A"}}]},{arrayName:[{fieldName1:{fieldName2:"B"}}]},{arrayName:[{fieldName1:{fieldName2:"C"}}]}]

db.collection.aggregate([{$addFields:{arrayName:{"$filter":{"input":"$arrayName","as":"a","cond":{$eq:[{$indexOfBytes:["$$a.fieldName1.fieldName2","A"]},-1]}}}}},{$match:{$expr:{$ne:[{$size:"$arrayName"},0]}}}])