Looking into this, setting BLOCK_LEVEL to false removes the problem. I assume it's set to true for nesting reasons or some such? Assuming the block level setting is important, I'd propose changing line 372 as follows to resolve the issue:
if ($tag[Codes::ATTR_BLOCK_LEVEL] && $tag[Codes::ATTR_TAG] !== 'footnote' && isset($this->message[$this->pos]) && substr_compare($this->message, '<br />', $this->pos, 6) === 0)