this is brilliant. iOS Deployment Target is the operating system level. if you add a new extension to an old app, it will have a deployment target (OS level) set to the highest that the Xcode version that you are using supports, by default. however, if your test device is at a lower OS level, the extension will not show and you won't see any errors reported. silent fail. this answer fixed it for me.
Topic:
Developer Tools & Services
SubTopic:
Xcode
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