The MC200, Roon core, and MS600 are all connected to the one switch.
The switch is connected to a router.
All devices have been rebooted.

This little ecosystem (with the exception of the Roon core) has been working without a hiccup for over a year.

The Roon core was added recently. Even with it powered off, the MS600 refuses to show itself on the network.

Source: Meridian MC200->MS600->PS Audio DS DAC
Linn Sondek LP12 Radikal Kore/Kuzma Stogi S/Soundsmith 'The Voice'
Phono: PS Audio GCPH
Pre: Wyred4Sound STP SE
Power: Krell FPB200
Speakers: Thiel CS2.7