It seems to be an odd quirk, maybe related to the forum software more than the website itself. I found that refreshing the page usually solves the issue. A bit annoying, but it isn’t completely broken.
As mentioned above, this is intentional. By default if you use the slug of a category as a hashtag, Discourse points to the category itself. The slug of the category is the name of the category that appears in the link to it, e.g Forum Feedback’s “slug” is “forum-feedback” as you can see from the link to it:
If you want to search topics with the tag lounge, then you will have to do tag:lounge in your search query.