db.r.find()
{ "_id" : 10, "a" : [ { "x" : 1 }, { "y" : 1 } ] }
{ "_id" : 11, "a" : [ { "x" : [ 1, 2 ] } ] }
{ "_id" : 12, "a" : [ { "x" : 1 }, { "x" : 2 } ] }

db.l.find()
{ "_id" : 0, "b" : null }

// When "r" is used as local collection, "a.0.x" doesn't match to null (expected):
db.r.aggregate({$lookup: {from:"l", localField:"a.0.x", foreignField:"b", as:"matched"}})
{ "_id" : 10, "a" : [ { "x" : 1 }, { "y" : 1 } ], "matched" : [ ] }
{ "_id" : 11, "a" : [ { "x" : [ 1, 2 ] } ], "matched" : [ ] }
{ "_id" : 12, "a" : [ { "x" : 1 }, { "x" : 2 } ], "matched" : [ ] }

// But when "r" is used as a foreign collection, all three documents are matched to null (unexpected):
db.l.aggregate({$lookup: {from:"r", localField:"b", foreignField:"a.0.x", as:"matched"}})
{ "_id" : 0, "b" : null, "matched" : [ { "_id" : 10, "a" : [ { "x" : 1 }, { "y" : 1 } ] }, { "_id" : 11, "a" : [ { "x" : [ 1, 2 ] } ] }, { "_id" : 12, "a" : [ { "x" : 1 }, { "x" : 2 } ] } ] }