It eventually resolved itself. Not sure why only one of them did that, nor why they all worked in TestFlight for several days before publishing to the App Store, but 🤷‍♂️

Xcode Cloud has the same problem for what it's worth. Changing the Xcode version of the workflow to beta4 allows the build to succeed, but apps built before beta5 crash on the latest iOS beta so... nothing works right now.