I think this might be related to FIX: category page improvements by yuriyaran · Pull Request #36062 · discourse/discourse · GitHub.
I miss the search function in the category drop-down menu. It disappeared on my site after the update today.
After reading the discussion on GitHub, I thought this wouldn’t be a problem. Apparently, it is for me. I think it’s because of lazy_load_categories_groups. When that isn’t active (which was a bit complicated now that the setting is hidden, as I don’t have server access 
), the search is available again (and all 3 subcategories are visible at the bottom)
For comparison, I configured lazy_load_categories_groups in my dev install and then took a screenshot before and after the update. Here you can see how searching was possible. [1]
The search also returned categories that were not displayed in the drop-down menu.
After the update, the only option to get to “test12” is to use the ‘all’ link at the bottom right, which takes you to the category page where you can then click on the desired category and then click the button to edit it. This makes switching between two categories that you want to configure considerably more complicated. Of course, search is not only missing when you edit categories but also everywhere else where the drop-down is shown. But I first noticed it while editing when I wanted to reproduce the bug. Navigating from one category to another via the category page is not as convenient as searching directly but requires fewer steps than navigating between the edit pages.
Once I leave the group for which I enabled lazy_load_categories_groups the search option in the drop-down is back:
I disabled the
max-heighton.select-kit-collectionso the whole drop-down is visible ↩︎