Update: validation of a prior build that was accepted by and available in App Store Connect as of two weeks ago failed validation with the same "Missing Purpose String" error. It was the same archive, prepared and successfully uploaded two weeks ago. We used Manage Version and Build Number option in Xcode 13.1 so that they'll be incremented and won't conflict with the prior upload. This seems to be a bug in App Store Connect. Does anyone experience a similar problem?

Thank you for the tip about adding NSHealthShareUsageDescription and NSHealthUpdateUsageDescription to other extensions! Could you please share what were the extension types you updated with these? We have several extensions (for Siri, today widgets, watch, iMessage, notifications, etc) We did try to add these to our watch app extension, but it was reported as an error by App Store Connect.

UPDATE: following the tip form andybodymatter, we've added NSHealthUpdateUsageDescription to our watch extension (not watch app extension that we did prior) and this fixed the validation problem. Thank you everyone who responded!