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 see the problem in the attached debugger screenshot:
The relevant code is here https://github.com/NativeScript/mlkit/blob/c7ee4981e0457293e655d75dbf83b643fac48606/packages/mlkit-core/index.ios.ts#L205
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:
https://github.com/NativeScript/mlkit/blob/c7ee4981e0457293e655d75dbf83b643fac48606/packages/mlkit-core/index.ios.ts#L205