Steps to reproduce the issue

Having a form defined like this:

<?xml version="1.0" encoding="utf-8"?>
<form>
    <fields name="custom_fields" />
</form>

Note: I've only added the fields section to explain how this issue happens. custom_fields is just an empty fields section there to allow field injection.

Before you could do this to add a field to a form fields section:

		$xml = '<field type="text" name="test-field" label="Test field" />';

		$form->setField(new \simpleXmlElement($xml), 'custom_fields');

And retrieve the field like:

$field = $form->getField('test-field', 'custom_fields');

Which was returning the correct field.

After this:
#14737

It looks like we are now forcing that a fieldset exists on the fields section:

//fields[@name="custom_fields"]/fieldset[@name="default"]

because the new $fieldset param added to the function:
https://github.com/joomla/joomla-cms/pull/14737/files#diff-ed54af5d39d21ed9a84d28eba903bd5fR1082

So the field is never return.

Expected result

I'd expect B/C. So if fieldset was not required before now it shouldn't. Before this XML worked:

<fields name="custom_fields">
        <!-- fields here -->
</fields>

Actual result

Now fieldset is mandatory so you need an structure like:

<fields name="custom_fields">
    <fieldset name="default">
        <!-- fields here -->
    </fieldset>
</fields>

System information (as much as possible)

Joomla v3.7.0

Additional comments

See #14737