Use the following code to test:

$xml   = '<form>
            <fields name="level1">
                <fields name="level2">
                    <field name="field1" type="text"/>
                    <field name="field2" type="custom"/>
                    <field name="field3" type="custom"/>
                </fields>
            </fields>
        </form>';

$array = array(
            'level1' => array(
                'level2' => array(
                    'field1' => 'field_value',
                    'field2' => array(
                        'a' ,
                        (object) array('b' => 'B' ,'c' => 'C')
                    ),
                    'field3' => (object) array('a' => 'AA' ,'b' => 'BB')
                )
            )
        );

$data  = Joomla\Utilities\ArrayHelper::toObject($array, 'JObject');
$form  = JForm::getInstance('JFormBindTest', $xml);
$form->bind($data);

echo '<pre>';
print_r($data);
print_r($form);
echo '</pre>';

@vietvh Please check this out and cross test against #3737 (comment)

Can This ne tested somehow? E.g. With a extension that don't work bevor but after that patch?

@zero-24 I am not particularly seeing any such extension, however the referenced PR (#3737) has changed the behavior of JForm bind method in this regard, as a result of which few extensions are likely to break.
That issue is fixed here along with the fix made by #3737. Therefore both the test cases should pass now.

The comment #3737 (comment) by @vietvh mentions some issue regarding previous change.

@izharaazmi : the new change in this pull request worked fine with my code. At least from what I tested.

The common form that I've seen from core Joomla components and many 3rd extensions is using this structure:

1. form > fieldset > field
2. form > fields > fieldset > field

From your use case, you are nesting 2 same tags (form > fields > fields > field). This is the first time I see this therefore I am a little bit curious, why did you have to use that instead of the common pattern 1 and 2 above?

@vietvh You are right about the patterns used commonly. But that is not restricted by Joomla, and the nesting like fields > fields > fields > ... at any level creates the form elements at the same depth.

I'd also like to mention that fieldset does not affect the nesting of input fields in the output.

Therefore to replicate the nesting issue in minimal code I just used such nesting. Hope this answers your query.

@vietvh If you tested the PR with your code and the given test case. Please update the test result here.

Worked fine for me, this patch fixes the following:

1. It does not generate PHP warning (as mentioned in #3737)
2. It does not break the current JForm behavior (like #3737 did)
3. It allows nested form fields as in test case above.

@Kubik-Rubik The PR for the extended issue reported after merge in #3737 is here. Kindly have a look at your convenience.

I have tested this item successfully on 64bc802

I have tested this successfully. The tags were not working before, they are working now.

Setting to RTC as we have 2 successful tests.