I know that Mastodon is not the same as ActivityPub, but I don't know how can it be treated seriously if it allows disappearing replies. Whatever we write will disappear after some time. Sometimes. Because sometimes not. Maybe it's an implementation problem, I don't know, but it was one of my two reasons for my exit from Mastodon.
It's literally a feature - individual users can set post auto-deletion on their instance. Because a government could suddenly start firing people from their jobs for some shitpost they made a decade ago.
But you have the fact that this is the Internet, and somebody will have archived your post no matter what you or your host instance does. So you can rest assured that whatever you wrote on Mastodon is out there somewhere...
I think it’s ultimately up to your instance whether it keeps your posts indefinitely or not. I think most do, but others might delete posts after a period of time, in which case they should mention this to their users (on their ‘About’ page, for example). Personally, I can’t say I’ve encountered this problem, but then again I’ve mostly used Pleroma (which is a different program implementing the ActivityPub protocol, like Mastodon).
