[{"tasks":[{ownerIds:[123,456]}]}]

db.collection.aggregate([{$addFields:{"isOwner":{"$in":[123,{"$arrayElemAt":[{$map:{input:{$filter:{input:{"$objectToArray":{"$arrayElemAt":["$tasks",0]}},cond:{$eq:["$$this.k","ownerIds"]}}},in:"$$this.v"}},0]}]}}}])