I wonder if this app is also an “app clip” - the small size applets that can bypass download size restrictions (and maybe, because of that, other restrictions).

How big is the installed size, and can you go to settings and delete app data before deleting the app? You may also check to see if it’s installed on other devices connected to your iCloud account or if you have any devices you never disconnected including Apple Watch as they have a watch app.

My wife works for macrumors and she once found an incomprehensible HomeKit bug and we had an Apple engineer come to our house to diagnose (we lived nearby to Cupertino)

It turned out to be a bug in the legacy HomeKit after upgrading on the backend. Totally opaque to the end user.