Video not available

Had the same issue.

Disabling IPv6 (which you can do in System Setting/Network on the Mac, but must do at router level for the Apple TV, and probably on other devices such as smart TVs) solved it.

Connecting through a VPN (which happened to prioritize IPv4 over IPv6) or connecting to my neighbors' WIFI (with their agreement, their network is IPv4 only) also solved it.

Apple system status page says everything is OK, on Apple - Support - System Status

Please send feedback to Apple on Product Feedback - Apple so they get aware of the problem.

This may indicate that the TV is not receiving a strong enough Wi-Fi signal when streaming content. Please do not confuse signal strength with signal speed, they are 2 separate subjects.

You can easily measure the signal strength with an iPhone or iPad with a tool like iStumbler, this is an app that is designed to measure strength. If you find that your signal's strength is marginal or low then it may be time to upgrade your router or network to fix that problem.

Select apps on smart TVs may misbehave for a variety of reasons. Troubleshooting that would differ by platform (smart OS) — so, what is your brand and model smart TV?