moodleform addGroup separator does not work same as in previous version

While updating plugins used at moodle.org for moodle 3.5, I have discovered a regression in the Boost theme.

I have a form code like this:

    public function definition() {

        $mform = $this->_form;

        $mform->addGroup([

            $mform->createElement('checkbox', 'foo1', null, 'Foo1'),

            $mform->createElement('checkbox', 'foo2', null, 'Foo2'),

            $mform->createElement('checkbox', 'foo3', null, 'Foo3'),

        ], 'foos', 'Select foos', '<br>', false);

    }

In Clean theme and in Boost 3.4, the checkbox elements are displayed as stacked vertically, using the <br> as the separator.

In Boost 3.5 however, the separator seems to be ignored and the items are displayed next to each other.

Steps to reproduce

1. Go to a course and turn editing mode on
2. Add a new SCORM activity
3. Expand the Appearance section
4. Click "Show more..." link
5. Observe the checkbox options for the "Options (Prevented by some browsers)" field

Expected behaviour: Options are supposed to be stacked vertically

Actual behaviour: Options are displayed next to each other