Since it doesn’t seem to work with original HTML, I’m now using a different approach:
[Test]:|This is a tooltip as a test.
add_filter( 'glossary_tooltip_html', function( $tooltip, $excerpt, $photo, $post_id, $noreadmore ) {
$HTML = str_replace("|","<br>",$tooltip);
$HTML = str_replace("[","<b>",$HTML);
$HTML = str_replace("]","</b>",$HTML);
return $HTML;
}, 10, 5 );