After changing template, mail is plaintext independent if configuration

**Describe the bug**
Due to import of attendees via CSV directly, we fill a custom reference for each attendee.
This reference shall be seen in ticket mail and ticket PDF.
After adding `Your reference is: {{ticket.extReference}}` in line 4 in the TICKET_EMAIL template, all tickets are sent in plaintext.

**To Reproduce**
1. Create organization and event
2. Open the attached CSV [alfioInvitation.csv](https://github.com/user-attachments/files/17608407/alfioInvitation.csv) and set a valid mail address and save the file
3. Select event and click Actions -> import attendees
4. Drop the CSV file from above
5. Select an existing category (Check: send tickets via email - this should be an HTML mail)
6. Click Upload
7. Go to Configuration -> Name of organization -> Templates -> TICKET_EMAIL
8. Add the line from above and save the template
9. Go back to event detail
10. Click reservations and select the imported reservation id and scroll down to the bottom
11. Click Send tickets vie email, check the test attendee and click send. This email is plaintext.

**Expected behavior**
HTML ticket email, but with the additional sentence.

**Screenshots**

**Desktop (please complete the following information):**
- OS: Windows
- Browser: Firefox
- Version: 129.0.1 64-bit

**Additional context**
- Docker container: alfio/alf.io:latest
- Alf.io version: v.2.0-M5

Edit:
I found [Discussion 1292](https://github.com/alfio-event/alf.io/discussions/1292), but I restarted the docker container without change in behavior. And I don't want to change the styling - only the text.