notify-send sometimes stops working until mako is restarted

I've noticed that sometimes notify-send stops working (ie no mako notification appears) until mako is restarted using SIGTERM and then executing mako again.

Using:

```sh
dbus-run-session river
```

to run my graphical session

I do `riverctl spawn mako` in my River (classic, 0.3.14) init to run mako.

libnotify is version 0.8.8

mako is version 1.10.0

OS is Artix Linux x86_64; packages are up to date.

notify-send is being invoked by various programs, including me opening my terminal emulator and running notify-send on there.

Notifications sent by some applications are appearing (Firefox and Discord), which I assume are not using libnotify. But trying to do `notify-send` does nothing. `notify-send` completes with exit code 0/no errors, but no notification appears.

Any ideas? This might not be a mako bug, but given that restarting mako fixes it, I thought here would be a good starting point for figuring this out.