I appreciate Urquhart1244's help in the dozen or more threads related to this bug (software defect).
I just bought an Apple 4K 32gb (A2169) and found the same problem reported widely: Aerial screensaver ONLY shows "Downtown LA (at night)." Configuring Settings, General, Screen Saver to Refresh: Daily, Start After: 5 minutes, General/Sleep After: never ... the Apple TV did not update with any new or additional screensavers for over seven days. (Yes, I left the Apple TV running for seven days continually, with TV and AVR turned off.) I have very little on the Apple TV, utilizing only about 550MB of storage consumed. NO downloaded games. (Someone asked, "how do you know how much storage is used" in another post. Simple. Settings, General, Manage Storage ... then do the math. Unknown how much TV OS and system cache consume, but I strongly doubt that it uses 31GB.)
In other posts I saw repeated references to a Themes option when Aerial is selected. This stumped me as my Apple TV (shipped with then-current TV OS 15.5.1) did not offer this under Settings, General, Screen Saver, even after selecting Apple Photos, Landscape, letting that run for a day, then switching back to Aerial. Even after Settings, System, Restart -- still no option for Themes. Very odd; I wondered if this only appeared if you had 4K UHD or HDR equipment connected. But keep reading....
Two days ago there was an update available to 15.6. I updated to that, hoping that would be a last alternative to a full reset. Nope, the Aerial Screen Saver was still stuck on "Downtown LA" and no others downloaded overnight. No Themes option, either. I finally cursed under my breath and performed a Settings, System, Reset as some others suggested. As I only have 5-6 subscriptions on my Apple TV, this only wasted about 15 minutes of my time to re-activate them, in addition to the time to perform the reset.
Voila! Now the Apple TV 4K has FOUR Aerial screen savers: "Passing over Horn of Africa," "Burnt Canyon area of Grand Canyon," "Dubai, and "Humpback Whale." Basically, one from each Theme. And the Settings, General, Screen Saver, Themes option is now present. This constitutes one defect (potentially with the installed OS image for new units), the Themes option was not present on initial set up of the Apple TV, and not present after a TV OS update. I wonder if this is why only one, default installed Aerial screen saver ("Downtown LA") was present on initial setup. (I've tested software for a living over 20 years total, so I don't use the "bug" term cavalierly.)
I've selected the Apple Photos, Landscape, option and invoked it, performed a Restart, then switched back to Aerial. I'll again run the Apple TV for several days and reply here with results.
It seems that only Apple TV 4K owners report this issue. I suspect this is a defect with the Apple TV 4K since I used an Apple TV HD (4gen) for five years, last used two weeks ago with 15.5.1, and counted 13 or more Aerial screen savers. (I particularly love the new ones from Scotland, Iceland, Chile, Grand Canyon.) Since I don't have suitable UHD 4K TV or AVR connected yet, I upgraded to benefit from the faster chip, future-proof video capability, and deep discount on Amazon Prime Day. I'm not yet sorry that I upgraded, but I am disappointed in Apple for offering no solution to this defect after more than two years of complaints from customers. C'mon, Apple, you can do better.