You can see the problem in the attached debugger screenshot:
The relevant code is here
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if (this.detectionType === DetectionType.None || !this.hasListeners?.(MLKitView.detectionEvent)) { |
It looks like the function is called with the wrong context. The "this" pointer seems to not point to the MLKitView object but rather a different NativeScriptGlobal object, like you can see in the debugger
this.detectionType = undefined for it, hence the onDetection method gets never called.

You can try with the demo app at:
https://github.com/cjohn001/mlkit-test/
Just run on IOS device and set a breakpoint to:
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if (this.detectionType === DetectionType.None || !this.hasListeners?.(MLKitView.detectionEvent)) { |