I just tested this with Discourse version 2.9.0.beta3 and was unable to reproduce the issue.
Clicking on an image in a topic brings up a Lightbox containing the image, and the HTML shows that the lightbox class is being applied to the image as well.
I’m accessing Discourse through Google Chrome Version 99.0.4844.82, and the Discourse site I tested this with is being hosted through Digital Ocean.
Hopefully this helps provide some more insight on what might be causing this issue on your site.