Can you try reproducing on https://wpmaster.demo.civicrm.org/ (I did try and wasn't able to).
Looks like its a environment issue. Could you please check your htaccess or server config? Or WP permlinks?
Unless writing an extension implementing token processor its not possible to fetch the participant id in reminders.
If you familiar with smarty and the contact will have only one participant for a event than you can try below snippet in your reminder message template