Also: if hashtags are at the end of a post, they will be displayed in a smaller font below the post rather than as part of it.
You may be following one of these tags that are in the smaller font?
@FediTips Well, that didn't take long: https://fosstodon.org/@favogiuseppo@lemmy.world/115553260852664081
On @pachli (the main way I use Mastodon) it looks like the screenshot in my timeline.
Not boosted, no hashtags, and I don't follow the poster. However on the web interface of Mastodon it does show the linuxmint hashtag, which I do follow, so that explains it. I guess the problem is that #Pachli doesn't show the hashtag for some reason?