I also have the same problem; weak 5Ghz coverage.
I’ve got older Apple devices - iPhone8, iPhone 6S, iPad mini 2 and a MacBook, all are fine but the XS is having coverage problems which results in failing back to 3/4G. Often I am unable to connect back to WiFi due to the password being requested/rejected.
I raised a case with Apple which was escalated to a senior engineer. They added some logging profiles to the handset, the results of which and video showing the issue were sent back. They advised there was no issue found with the software and asked me to return the phone for a replacement.
I’m now using the replacement which exhibits the same issues and I have updated Apple.
Let’s hope they find a solution soon!