This is/was driving me nuts... after cleaning virtually all warnings, deleting identifiers, toggling automatic provisioning in Xcode and a million other things, the advice above may have been the key. Here is what I did:
From Xcode, perform an Archive of the iPhone app (I have the main app, widgets, and a companion watch app with its own widget extension as well (probably source of the problem)).
After the Archive is created, perform a Validate.
After Validate, perform a Distribute, choose Custom, choose Release Testing, choose App Thinning "All".
After this, it appears to be working again... I got a TestFlight notification on my phone. Fingers crossed for Xcode cloud build, next.
Update: Xcode Cloud did NOT work. I thought the above might reinitialize something but apparently not. Still failing on my watchOS archive:
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Run command: 'xcodebuild -exportArchive -archivePath /Volumes/workspace/tmp/0ad1cd3c-4b88-48c6-b61f-ace8da289cdd.xcarchive -exportPath /Volumes/workspace/adhocexport -exportOptionsPlist /Volumes/workspace/ci/ad-hoc-exportoptions.plist '-DVTPortalRequest.Endpoint=http://172.16.55.65:8089' -DVTProvisioningIsManaged=YES -IDEDistributionLogDirectory=/Volumes/workspace/tmp/ad-hoc-export-archive-logs -DVTSkipCertificateValidityCheck=YES -DVTServicesLogLevel=3'
New ideas welcome!