Reproduction steps:
1. Open the attached project.
2. Build the project.
3. Open the exported project with XCode.
4. Archive the build ('Product > Archive').
5. Select the archived build and click Export.
6. Select Ad Hoc and Click Next.
7. Make sure the option "Rebuild from Bitcode" is checked and click Next.
8. Select "Automatically manage signing" and click Next.
- If you'll encounter problems with certificate: http://www.ironpaper.com/webintel/articles/how-to-share-an-ios-distribution-certificate/
9. Wait for the compilation to end.
10. Click Export and choose a place to save.
11. Go back to XCode and click on Windows > Devices and simulators.
12. Select your connected iOS device, scroll down and click on the rightmost +.
13. Select the previously exported IPA file.
14. Launch the application on the iOS device.
15. Wait for the application to crash.
16. Go back to the Window > Devices and Simulators.
17. Select the application StacktraceBugReproduction, click on the gear symbol in the bottom and select "Download container...".
18. Save it.
19. Go to that location, click right on the container and select "Show package contents".
20. Go into App Data/Documents and open log.txt.

Actual behavior:
- This file shows the message and stacktrace received by the unhandled exception callback. Observe that there's only one method logged situated in the .NET API.

IL2CPP
ArgumentNullException: Argument cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[System.Object,System.Object].TryGetValue (System.Object key, System.Object& value)

Mono
ArgumentNullException: Argument cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[System.String,System.String].TryGetValue (System.String key, System.String& value)
StacktraceBugReproduction.ThrowException ()
StacktraceBugReproduction.MethodCallB ()
StacktraceBugReproduction.MethodCallA ()
StacktraceBugReproduction.OnEnable ()
UnityEngine.UnhandledExceptionHandler:HandleUnhandledException(Object, UnhandledExceptionEventArgs)

Reproduced with:
2017.1.4f1, 2017.2.2p4, 2017.4.3f1, 2018.1.1f1, 2018.2.0b4, 2018.3.0a1

Tested with:
iPhone 8 iOS 11.0, iPhone 5s iOS 9.3.3