Filed as FB24619522 — sysdiagnoses from both devices and the live logs are attached to the report.
We ship WidgetKit complications, and on the watchOS 27 betas (24R5360a, paired with iOS 27 24A5430a) they die every time the app gets updated while they're on the active face. Within seconds they turn into empty placeholders and never come back on their own — opening the app, switching faces or reloading timelines from the app does nothing.
The watch logs show the extension being killed during the install (normal), and after that watchOS simply can't launch the new copy any more:
runningboardd: <OSLaunchdJob | handle=…>: start succeeded, info=spawn failed, error=111: Invalid or missing Program/ProgramArguments
chronod(ChronoKit): Reload failed; 0 retries remaining: … "Unknown extension process"
Only two things fix it, and both restart chronod: rebooting the watch, or switching the watch language and back (iPhone › Watch app › General › Language & Region, then give it a couple of minutes). Our second extension in the same bundle, which wasn't running during the install, came back fine — so this looks like an OS bug rather than something we can work around in code.
iOS 27 widgets seem to have the same problem (this thread, and iPhone in Canada wrote it up on beta 4), and someone posted the identical log lines back in March.
FB24619522 — the full story, both sysdiagnoses and the live logs are in the Feedback.