push element issue for a schema with an array of arrays

[RoosterH]

Yes, I am using 5.10.5

Do you want to request a feature or report a bug?
report a bug or maybe I just defined the schema wrongly?

What is the current behavior?
I want to define an array that contains multiple arrays, such as:

runGroupEntries[['userId1', 'userId2', 'userId3'], ['userId4', 'userId5', 'userId6], ...]

Here is my schema:

runGroupEntries: [
        [
            {
                type: mongoose.Types.ObjectId,
                ref: 'User',
                require: true
            }
        ]
    ]

The way I initialize the array:

for (let i = 0; i < numGroups; ++i) {
    event.runGroupEntries.push(undefined);
}

From Atlas, I can see it correctly as:

After push the first element to the array using the following:

event.runGroupEntries[runGroup].push(userId);  //runGroup = 0

I got

runGroupEntries: [ [ null, [Array] ], [ null ], [ null ], [ null ], [ null ] ],

You can see the pushed "userId" is showing as an array.

After pushing the 2nd element. The array became:

runGroupEntries: [ [ null, 5f5c1d95e4f678ce190d5624, [Array] ], ....]

The first pushed element became userId now but the 2nd element is an array.

If the current behavior is a bug, please provide the steps to reproduce.
Please use above lines to reproduce.

What is the expected behavior?
expecting all the elements are "userId" instead of seeing an array for the latest pushed element.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
"mongoose": "^5.10.5",
MongoDB v4.2.6
Node.js v12.16.2

[RoosterH]

Tried something different and found another issue.

I tried to replace the first null element with a real userID:

if (	event.runGroupEntries[runGroupIndex].length === 1 &&
	event.runGroupEntries[runGroupIndex][0] === null
	) {
	event.runGroupEntries[runGroupIndex][0] = userId;
} else {
	event.runGroupEntries[runGroupIndex].push(userId);
}

After re-assignment, I did a print out. The null element indeed been replaced with userId:

 runGroupEntries: [
    [ '5f5c1d95e4f678ce190d5624' ],
    [ null ],
    [ null ],
    [ null ],
    [ null ]
  ],

However, after I did save, I am using transaction BTW

await event.save({ session: session });

The userId is not showing in MongoDB

All other information is saved but not the userId in the runGroupEntries.

EDIT:
replaced event.runGroupEntries[runGroupIndex][0] = userId; with

event.runGroupEntries.set(0, userId);

UserId is now been saved to MongoDB but still shows it as an array: