As it turns out, it's only doing it in two places, not all. I did copy/paste from a document, so it must be something glitchy in the cleanup from Word formatting. I'll just delete each instance of the problem and retype the hashtag manually.
I found a few other problems where I kept getting error messages and it wouldn't let me save my drafts. These were instances where I'd quoted material with special characters - well, they were translated into emojis by Word, as I finally figured out - but the editor wasn't able to highlight the problems for me, so I had to sort it out using trial and error. Maybe the to-do list could include "remove emojis" in the Word cleanup code.