Does version 5.27 break links to live and test-drive pages for contribution pages?

Just upgraded a WP site to Civi 5.27.1. From ‘Manage Contribution Pages’ the links as given to Live and Test Drive for a given contribution page appear broken – i.e. they create a white screen indicative of a fatal error. The URLs provided are:

Live Page: https://www.example.org/wp-admin/admin.php?civiwp=CiviCRM&q=civicrm%2Fcontribute%2Ftransact&reset=1&id=1

and

Test Drive: https://www.example.org/wp-admin/admin.php?civiwp=CiviCRM&q=civicrm%2Fcontribute%2Ftransact&reset=1&id=1&action=preview

If I edit these URLs to replace "civiwp" with "page" they work fine.

Bug, or am I doing something wrong?

Seems to be related to this announcement: https://civicrm.org/blog/kcristiano/civicrm-526-and-wordpress-important-notice

Update: nothing in the Civi log.