Just tried cycling with variations. Neither restarting Roon Server or bouncing the Antipodes, with the backup NAS on or off or disconnected from the network, makes a difference.
When the system first started with this type of behaviour, it occurred only occasionally and was resolved by ensuring the backup NAS was turned on. The frequency of occurrence gradually increased over roughly two years (fortnightly - weekly - every few days) and could usually be resolved by turning on the NAS and restarting Roon Server or in come cases bouncing the Antipodes. Now, turning on (or off) the NAS seems to have no effect.
It seems odd that reinstalling Roon Server seems to resolve the issue, yet the behaviour starts recurring within a few days. I thought it might be a setting I change after the reinstallation is tested (backup, Tidal) that might causing the issue… but I don’t have enough into to make even a vague correlation. Probably unlikely anyway.
No progress at my end.