Apple TV 4K failing to connect to library with Home Sharing

I have a problem with Apple TV 4K and Home Sharing which has just started out of nowhere.

To set the scene, I have three other Apple TV units in the house that are third generation, and one single 4K model. All of them were previously working with Home Sharing, but in the last couple of days, the Apple TV 4K model keeps saying that it cannot find any libraries.

It is clearly not the case, as the iMac which I use as my home server has Home sharing switched on (otherwise, none of the other devices would be able to access content either). In addition, it is definitely not a home network connectivity issue, as every other service on the Apple TV 4K works, for example YouTube, Netflix, Arcade.

• I have not made any changes to settings anywhere.
• I have also tried logging out and logging back in to Home Sharing on the Apple TV 4K, but this has achieved nothing.
• Similarly, I have disabled and then re-enabled the Home Sharing option on my iMac. Note that the iMac is running Catalina, and the setting for Home Sharing is in System Preferences, and therefore does not require the TV app to be running for sharing to work.
• I have also tried a full restart on the Apple TV 4K device, which also does nothing.

Given that everything is logged in correctly, on all devices, and connectivity is sound, what's on earth is happening with the Apple TV 4K where it cannot find Home Sharing libraries but all older versions of Apple TV have no issue whatsoever?