How to reproduce:
1. Open the attached "XRRefPtRepro.zip" project
2. Open the "XRRefPtRepro" Scene
3. Build to UWP (don't forget to check the Development build and Script Debugging checkboxes)
4. Open the Visual Studio Solution file, located in the built project's directory
5. Select the target: Remote machine with Release and x86 options
6. Build and Deploy to Hololens
7. When app starts move around -> then move backwards
8. Close the App -> see the Output log

Expected result: All the trackables are correct and no Exceptions are being thrown
Actual result: "InvalidOperationException: Trackable 445AD0065803FB0A-DC9E932758067EB6 is being added but is already in the list of trackables" exception is being thrown

Reproducible with: 2019.4.6f1 (2.0.3), 2020.1.0f1(3.2.0), 2020.2.0a19(4.0.2)

Notes:
- The bug appears on Hololens 1 and Hololens 2
- Could not test on 2018.4, because Scripting errors
- Static markers have trackingState==TrackingState.Limited. Pulsing markers (growing and shrinking) have trackingState==TrackingState.None
- Reference points never have trackingState==TrackingState.Tracking
- As more markers are added (> ~3), earlier markers turn from TrackingState.Limited to TrackingState.None
- The Pose reported in Update for reference points which have trackingState==None is the right-handed equivalent of the expected Pose
- The Pose reported in Update for reference points which have trackingState==None is still changing