Copy Participants breaks on update

I've got a site running 5.33.1, Drupal 7:

• I can create event templates
• I set up some repeat events
• I add participants to the first event in the series
• Then select the events in the 'Manage Linked Events' (the source event and the empty target events)
• Actions > Copy Participants
• Nothing happens

The page just reloads the Manage Linked Events tab, no backtrace recorded in CiviCRM logs and nothing in Drupal logs.

I've tried updating CiviCRM to the latest version, issue remains. Even if it's a single participant to copy, same behaviour.