Leaf nodes validation not working correctly after upgrade mongoose >= 6.3.1

[skrtheboss]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.4.1

Node.js version

16.15.1

MongoDB server version

5.0.0

Description

After upgrading mongoose ta a version grater than 6.3.0, the validation stopped working in some conditions.
If a schema is created with leaf nodes, then the validation is only performed if the property is required.

Steps to Reproduce

const mongoose = require('mongoose');

const SubSubSchema = new mongoose.Schema(
    {
        from: {
            type: mongoose.Schema.Types.String,
            required: true,
        },
    },
    { _id: false }
);

const SubSchema = new mongoose.Schema(
    {
        nested: {
            type: SubSubSchema,
            required: false,
        },
    },
    { _id: false }
);

// DOES NOT FAIL - WRONG BEHAVIOR
async function runLeafTest() {
    const TestLeafSchema = new mongoose.Schema({
        testProp: {
            testSubProp: {
                type: SubSchema,
                required: true,
            },
        },
    });

    const TestLeafModel = mongoose.model('test-leaf-model', TestLeafSchema);

    const testModelInstance = new TestLeafModel({ testProp: { testSubProp: { nested: { from: null } } } });

    await testModelInstance.validate();
}

// DOES FAIL - CORRECT BEHAVIOR
async function runTest() {
    const TestLeafSchema = new mongoose.Schema({
        testSubProp: {
            type: SubSchema,
            required: true,
        },
    });

    const TestModel = mongoose.model('test-model', TestLeafSchema);

    const testModelInstance = new TestModel({ testSubProp: { nested: { from: null } } });

    await testModelInstance.validate();
}

await Promise.allSettled([runLeafTest(), runTest()]).then(([leafResult, nonLeafResult]) => {
    console.log({
        leafResult,
        nonLeafResult,
    });
});

Runkit 6.3.0 https://runkit.com/embed/r0jhjsjztl99
Runkit 6.3.1 https://runkit.com/embed/ylkawc9mlm1s
Runkit latest https://runkit.com/embed/s553idqmvs9x

Expected Behavior

Both leaf and non leaf nodes should be validated the same way.

[skrtheboss]

I think this is the commit that caused the regression.