Normally is not neccessary but try adding a views key to the return dict, like:

def action_cancel(self, cr, uid, ids, context=None):
 '''action_cancel() code here'''
    data_obj = self.pool.get('ir.model.data')
    dummy, view_id = data_obj.get_object_reference(cr, uid, 'module', 'view_external_id')
    
    return {
        'name': 'Cancel',
        'view_type': 'form',
        'view_mode': 'form',
        'res_model': 'mrp.production.cancel',
        'views': [(view_id, 'form')],
        'type': 'ir.actions.act_window',
        'target': 'new',
    }

change module and view_external_id to match the form view that you want to display