Joomla 3.9.14

<field
                name="test_fields"
                type="subform"
                label=""
                multiple="true"
                buttons="add,remove"
                groupByFieldset="true"
                min="1"
                max="5">
            <form>
                <fieldset name="section1">
                    <field
                            name="test_field"
                            type="text"
                            label="TESTFIELD1"
                            default=""
                    />
                </fieldset>
            </form>
        </field>

Go to the form.
Now there is one row with key 0.
Add one row with "+" button.
Now there are two rows with 0 and 1.
Remove first row with key 0 over button "-".
Now there are one row with key 1.
Add one row with "+" button.
Now there are two rows with key 1 and 1.