Vive Trackers Randomly Flipping Axis.

I think I solved the issue. Using the Enumerate Tracked Devices and specifying different devices types I was able to realise that one of the trackers seems to be recognised as a motion controller. The vive tracker up axis and the controller up axis seem to be different.

All I had to do to get both working as trackers is going to the SteamVR overlay, right-clicking on the tracker icon (any will do) and clicking on Manage Vive Trackers.

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A window opens and in there make sure that for each tracker you select a role that isn’t Held in hand, in my case I choose waist for both.

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Now with that setup go back to UE4 and try to run the EnumerateTrackedDevices with the DeviceType enum set to Other. You should see the right number of devices (two in my case) and if you use either GetDeviceWorldPose or GetTrackedDevicePositionAndOrientation the trackers should face both in the same direction.

Hopefully, it also works for you.