google translation

On line 68, change the field name suffix from 'X' to '0'.
Then the nested subforms (one within the other) begin to form normal JSON for writing to the database.

Simple Joomla 4 plugin (for test).
https://disk.yandex.ru/d/hi6n5Xu4-r7_zQ

See tab "Publishing" (down in fieldset "Metadata") by Article creation or edition.

You have to explain what you tries to fix, your original issue.

The changes here is wrong. And cannot be accepted.

You have to explain what you tries to fix, your original issue.

Try installing the plugin. Link above.

Without the suggested fixes, nested subforms are not correctly stored in the database. As a result, they cannot be used.

I can copy the plugin code here, but there will be a lot of code.

Copy it?

No need whole plugin, enough just XML for subform.

But, I think you talking about this #35721 issue.
It was already fixed, you have to wait for 4.0.4

No need whole plugin, enough just XML for subform.

main.xml

<?xml version="1.0" encoding="UTF-8"?>
<form>
	<fields name="metadata">
		<fieldset name="jmetadata">
			<field
				name="quest"
				type="subform"
				label="Поле quest"
				description="Описание поля quest"
				multiple="true"
				buttons="add,remove"
				layout="joomla.form.field.subform.repeatable"
				formsource="/plugins/content/metatitle/subforms/guestions.xml"
				/>
		</fieldset>
	</fields>
</form>

guestions.xml

<?xml version="1.0" encoding="UTF-8"?>
				<form>
					<field name="question" type="textarea" cols="40" rows="8" label="Вопрос:" />
					<field
							name="answers"
							type="subform"
							label="Поле answers"
							description="Описание поля answers"
							multiple="true"
							buttons="add,remove"
							min="3"
							layout="joomla.form.field.subform.repeatable"
							formsource="/plugins/content/metatitle/subforms/answers.xml"
						/>
				</form>

answers.xml

<?xml version="1.0" encoding="UTF-8"?>
						<form>
							<field
								name="answer"
								type="text"
								label="Ответ:"
								description="Описание поля answer" />
						</form>

yeah, well, I think it the same as #35721
and it already fixed,

You can try to test on latest 4.0-dev branch, or using nightly-builds https://developer.joomla.org/nightly-builds.html

Closed in reason stated above. Thanks for PR.

Don't work.

Before test:

1. Update Joomla from 4.0.3 to Joomla ‎4.0.4-dev‎4.0.4-dev (https://developer.joomla.org/nightlies/Joomla_4.0.3_to_4.0.4-dev-Development-Patch_Package.zip)
2. Updated plugin: https://disk.yandex.ru/d/tgSogz6hF6AjZA
   main.xml

<?xml version="1.0" encoding="UTF-8"?>
<form>
	<fields name="metadata">
		<fieldset name="jmetadata">
<!-- new code from: https://github.com/joomla/joomla-cms/pull/35726#issue-1014053544 -->
<field
        name="subform_parent"
        label="Subform Parent"
        type="subform"
        multiple="true"
        buttons="add,remove"
>
    <form>
        <field
                name="test_text"
                label="Test Text"
                type="text"
        />
        <field
                name="child_subform"
                label="Child Subform"
                type="subform"
                multiple="true"
        >
            <form>
                <field
                        name="child_text"
                        label="Child Field"
                        type="text"
                />
            </form>
        </field>
    </form>
</field>
<!-- end new code -->
		</fieldset>
	</fields>
</form>

3. before Test: (Ctrl + F5 - for browser cash clean an Article Edit and/or New Page)

5.

Пичалька =(

Test with Full_Package.
In some reason Patch_Package is incomplete.

Test with Full_Package.

It's work! =)

Спасибо! ))