I noticed after an upgrade to Events Manager 5.3.4 that the usual WordPress wpautop() wasn't being called on event descriptions anymore. Digging into it a little bit, I found the error was introduced in classes/em-event.php, at line 2129. The third argument to em_event_output_placeholder() is supposed to be assigned to $placeholder, but it's instead assigned to an unused variable name called $full_result. This results in none of the correct text filters to be applied to the event text.
Should be a simple fix, at least. Thanks!