When writing a comment, you can preview it. I didn’t see this feature when making a post.
Also, spoiler markdowns weren’t rendering in Boost when I tried using the menu insertion. I’ve seen other posts and comments with spoilers, so I’m not sure what’s happening.
Spoiler tags have been broken in Boost for a while. 😔 They render correctly in other apps, and also on the instance’s web UI. Hopefully this can get fixed soon.