hook_preprocess_fieldset seems not to be evaluated for fieldsets that use bootstrap-panel.html.twig

Yep, in hook_form_alter to change attribute label/heading/title:

$attr_name = 'attribute_size';
if(isset($form['purchased_entity']['widget'][0]['attributes'][$attr_name])){
        $attr_title = $form['purchased_entity']['widget'][0]['attributes'][$attr]['#title'];
        $form['purchased_entity']['widget'][0]['attributes'][$attr_name]['#title']=t('Select', [], ['context' => 'attributes']).' '.$attr_title.':';
}