Commit breaks showon functionality on my test box: a field with showon condition validated false (ie. field should not displayed) on the subform is displayed. If this field is a clickable button group (ie. BS style radio button), then they are not clickable.
For more details see #35721

Modify the above XML example to have a 2nd field with showon condition to test my previous comment:

<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"
                />
                <!-- Here is the field with showon -->
                <field
                        name="child_text2"
                        label="Child Field2"
                        type="text"
                        showon="child_text!:"
                />

            </form>
        </field>
    </form>
</field>

Showon will not work inside nested subfom until first save, that related to how subform and showon works.
It worked by accident, because a bug in subform.
It need to be fixed on "showon" side, but it not an easy one.

Test your original issue.

For showon test that PR #35735, but only with current.

I have tested this item ✅ successfully on f4b0dc2

@DelPoint Please visit here https://issues.joomla.org/tracker/joomla-cms/35726 and use this button:

I have tested this item ✅ successfully on f4b0dc2

@Fedik done as you wished. Sorry, didn't know that GitHub approval is not enough (I guess).

RTC

Thx