This is due to a recent change/fix to how placeholders are used in the description of an event. since you can use shortcodes combined with placeholders, placeholders on there aren't converted anymore since 5.3, only if within shortcode.
Here's a snippet to enable placeholders on your descriptions if needed (note that using ph's in shorcode may not work properly with this)
/**
* Parses the event description to convert placeholders
* @param string $result
* @param EM_Event $EM_Event
* @param string $placeholder
* @param string $target
* @return string
*/
function my_em_event_output_placeholder($result,$EM_Event,$placeholder,$target='html'){
if( in_array($placeholder, array('#_EVENTNOTES','#_NOTES','#_DESCRIPTION','#_LOCATIONNOTES','#_CATEGORYNOTES')) ){
$result = $EM_Event->output($result);
}
return $result;
}
add_filter('em_event_output_placeholder','my_em_event_output_placeholder',10,4);