Images do not appear in user warnings

[trichoplax]

Describe the bug
In my development environment (using Docker), adding an image to a warning seems to work from the moderator's perspective, but when the user next signs in they see only the alt text rather than the image in the warning.

To Reproduce
Steps to reproduce the behavior:

1. As a moderator, send a user a warning, including an uploaded image in the wording.
2. Sign in as the user.
3. See alt text instead of an image in the warning.

Does someone have a suitable account they could use to test this in either production or the dev server? I'm happy to be sent a warning on the dev server to see if the image shows up for me.