This applies also for the Apple Home App -- new architecture configuration.
It seems many people are having this issue. Why doesn't the Apple Home App's "Home" Screen show all the rooms?
In other related posts, some say to ensure that the associated devices in those rooms are set to display in the Home Screen (toggle selection under status). But in many cases, mine included, these are set to display, but the room still is not listed.
In my case, I have a leak detector / temperature sensor unit in the "Basement" Room. The temperature indication is part of the "Climate" temperature range on the Home Screen. The leak detector portion is not user-adjustable and is automatically set to display when a leak situation is occurring. Could it be that since the temperature indication portion is included in the Climate category, it is not included in the "Basement" on the Home Screen? And since there is no current leak/flood in progress, the leak detector portion is not showing either. And since that is all that is in my Basement it has decided not to include the Basement room on my Home Screen? That is unfortunate.
Similarly, I have a temperature/humidity indicator in my "Living Room". Both elements of this device are set to be included in the Home Summaries. The Living Room temperature is included in the Climate temperature range on the Home Screen. The "Living Room" is showing on my Home Screen. But under it, it is only showing the status of my Apple TV which is also in the Living Room. By selecting Living Room from the Home Screen (clicking on the little circle with the 3 dots (...) in it in the top right of the Home Screen, has a pop-up menu with the Rooms list at the bottom of it), it shows the Living Room temperature/humidity and Apple TV status. That is a lot to go through to get to this information that we requested to be on the Home Summaries.
Apple engineers/developers, please, this should not be so difficult. It should be that if you select an item for inclusion in the Home Summaries, it and its Room are listed in the Home Summaries. Maybe it would be best to have another option where we can select if we want particular rooms to be in Home Summaries as well. That way even if they have no current devices at least they can be listed there.
In addition, as a side issue, on my iPhone at least (iPhone SE 3rd Gen), a number of the icons on the Apple Home App Home Screen (e.g, the Speakers and TVs) are partially cut off on the right side. And on my Family Room screen (which has a power/light switch, with 3 scenes listed left to right), the right most Scene is substantially cut off. This should also be repaired. A little bit beyond the scope of this discussion, but it may be related overall to work yet outstanding.
Thanks,