The activity names auto-linking filter doesn't work in forum notification emails (2)

To reproduce:

1. Ensure that the activity names auto-linking filter is enabled
2. Post in a forum and mention the name of another activity in the course
3. Check the forum notification email

Expected result:
The forum notification email will contain a link to the activity.

Actual result:
The forum notification email does not contain a link to the activity. The link is only displayed when viewing the forum post in Moodle.

This issue was reported previously as MDL-55499, however it is still not working on https://learn.moodle.net/ despite the activity names auto-linking filter being enabled at activity and course level.