Check the video for repro steps. Although the category name is full in the preview box, it doesn’t appear in the actual post.
This is also happening with mentions.
Hmm, we should take a look at this one early next week @eviltrout
I fixed this issue for mentions last week in this commit FIX: Update preview when autocomplete is clicked (#10430) · discourse/discourse@b9891e3 · GitHub.
The fix for category autocomplete is pretty much the exact same. Here is a PR FIX: Clicking on category autocomplete row by markvanlan · Pull Request #10457 · discourse/discourse · GitHub.