I think it is a waiting game for a setting to be changed on their server. I have tried the same xip file and the new App Store install. I have deleted the derived data. I have installed macOS 26.2. I have reinstalled the SDK components. At the time of writing this latest Xcode release is still not mentioned in the release notes link provided by Albert above.

Hi Albert, Our company has a number of points (bug fixes) pending with one of the key problems resolved by the .tabViewBottomAccessory(isEnabled:) modifier added in Xcode 26.2. Could you offer a finger in the wind date when RC builds will be accepted? This will allow us to decide whether to submit without the aforementioned modifier fix before our own seasonal company code freeze and the December 19-26 dates that Apple has indicated to avoid important submissions.

Xcode 26.2 beta 2 builds seem to improve support for iOS 26.1 because I was no longer able to replicate the occasional fail to render the accessory view case. My tests were based on criteria associated with the isEnabled: criteria based on which tab was selected and other states.