The original setup was confusing. I ended up hacking some modules to debug why notifications didn't seem to be working, but eventually I got everything working. (I also wrote a module to sandbox Prosody using pledge and unveil: https://github.com/wahern/prosody-openbsd)

Then OpenBSD 7.1 was released, which upgraded Prosody from 0.11 to 0.12. It's roughly around that time that the iPhone Monal user stopped seeing notifications for the MUC. But the Monal app also updated around that time, so it's difficult to place blame. As far as I can tell notifications seem to be going out to the push server, but it's still a bear to debug and I haven't had the time to properly analyze things, let alone fix them.

I also installed macOS Monal on my young son's macbook so he could IM me while at work, but for some reason desktop notifications don't work at all for him, not even 1:1 chats, and neither pre- or post-Prosody upgrade. (They work for me on my mac mini, though.) We can still chat, but he has no idea if I replied unless he's monitoring the Monal chat window. It's most likely a client-side Monal or macOS issue, though, because Monal doesn't even appear in the Systems Preferences -> Notifications & Focus application list, and side-channel push notifications shouldn't even be involved.

Years ago I used Adium on macOS and ejabberd server-side and things seemed to work more smoothly, even when chatting with Google Talk users. But now Adium is no longer maintained and has fallen behind in extension support. In some ways the XMPP ecosystem seems to have regressed even for the simple stuff, let alone the more complex mobile and MUC support.