Steps to reproduce

1. Under Administration > Security > Site Security Settings turn on Force users to login.  
2. Under Site Administration > General > Advanced Features turn on Allow stealth activities.
3. Under Site Administration > Users > Permissions > User policies turn on Auto-login guests.
4. Create a course and enable Guest access.
5. Add an activity.
6. From the activity settings, under Common module settings > Availability, select Make available but don't show on course page.

Expected results

When sharing a direct link to the activity (such as https://mysite/mod/page/view.php?id=123), the activity will display.

Actual results

A user selecting a direct link to the activity is redirected to the main course page, with an alert message displayed at the top stating "Sorry, this activity is currently hidden." If the user keeps the browser open and selects the direct link again, the activity then displays as expected.