It's weird and I don't have an idea yet what could be behind it, but somewhere between 19-30 August hashtags stopped being received on posts coming in from some accounts. Doesn't seem to be one server that's the problem, or one hashtag. So far I find some Mastodon accounts that no longer have working hashtags in their posts, but most Mastodon accounts are ok...

Edit: it's the creation time of the post that matters, not the time it is received.
Edit edit: the issue is local to my server; on other servers the same posts, accounts and hashtags work fine.