Now that chat.sr.ht has support for push notifications I started receiving notifications from SaslServ (which always mentions my nick) even though it is muted. I know the problem is with push because I receive said notifications even when I'm connected to my university wifi, which blocks IRC ports. In fact, I think this only happens when I'm connected to my uni wifi, so maybe the problem occurs only when IRC ports are blocked.
Fixed in 37f968d5b030 ("firebase: don't show notifications for muted buffers").