When you start a hashtag to refer to another category or tag, if you start it as the first thing on a new line it does not bring up the autocomplete box. This is happening on my site, confirmed here on meta through screenshots below. This means that when I want to list categories one on each line, I have to first compose them on one line, then I hit enter.
They appropriately render, mind you:
But if I don’t know the path for a subcategory or a tag, I have to guess or compose on a previous line to get the correct path.
Example here, with other characters first:
Newline, hashtag first: