Cannot sort arrays using unwind or reduce mongodb aggregation pipline

I am having a problem sorting an array in a mongodb aggregation pipeline. I couldn't find an answer in stackoverflow so am posting my problem

I have some data in mongodb which has this structure

{
testOne:{ scores:[1, 5, 8]},
testTwo:{scores:[3, 4, 5]},
testThree:{scores:[9,0,1]},
........
testFifty:{scores:[0,8,1]}
}


Basically a series of many tests and the scores from those tests (this is a simplified example to illustrate the problem).

After a few hours banging my head against a wall I found I could only unwind two arrays, any more and no data was returned.

So I couldn't unwind all my arrays and group to get arrays of each test scores from the db

db.collection.aggregate([
{
{$unwind: '$testOne.scores'},
{$unwind: '$testTwo.scores'},
{$unwind: '$testThree.scores'},
{$group:{
_id: {},
concatTestOne: { $push: "$testOne.scores"},
concatTestTwo: { $push: "$testTwo.scores"},
concatTestThree: { $push: "$testThree.scores"}
}
}


To my frustration , even if I just did one unwind the resulting array did not sort properly see below:

db.collection.aggregate([
{
{$unwind: '$testOne.scores'},
{$group:{
_id: {},
concatTestOne: { $push: "$testOne.scores"}
}
{$sort:{concatTestOne: 1}},
}


The result was [1,5,8,3,4,5,....,9,0,1]. No sorting

So to get all the tests scores I used reduce to flatten the nested arrays resulting from the grouping stage with no 'unwinding' e.g.:

db.collection.aggregate([
{
{$group:{
_id: {},
concatTestOne: { $push: "$testOne.scores"}
},
{$addFields:{

testOneScores: {$reduce: {
input: "$concatTestOne",
initialValue: [ ],
in: { $concatArrays: [ "$$value", "$$this" ] }
}
}
}


Once again the resulting arrays do not sort. Can anyone tell me what I am doing wrong. The arrays are large (length approx 3500) is it just mongoDB aggregation doesn't handle large arrays ?

Many thanks for any comments I have spent a lot of time trying to sort my arrays and noting works so far

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