Inconsistent access to Bonjour services on sleeping devices
I eventually tracked this down to the behaviour of one of my Powerline adapters, specifically a TP-Link TL-WPA4220. For whatever reason, multicast traffic does not appear to correctly pass through the WiFi on this device, even with the latest firmware. 

TP-Link have a related technical note on this, but it doesn't seem to apply to my particular device. In the meantime, I've just left the WiFi turned off on this adapter (still using it for two wired devices, including the 3rd gen Apple TV), and things seem to be working better again.