this is similar to https://forum.inaturalist.org/t/tags-for-links-no-longer-hidden-in-about-project-pages/14273. both appear related to a recent change in the system to address this feature request: https://forum.inaturalist.org/t/easier-access-to-text-formatting-in-comments/944.
the problem here is that the html syntax in your comment is invalid. the html parser in the past seems to have tolerated and fixed the invalid html. but the new markdown parser seems to be intolerant of invalid html.
in your example above, the proper html syntax would be:
<a href="https://www.inaturalist.org/observations/51259149">this observation</a>
if you fix the syntax, then all will be fine in the world.