I have been trying to solve this problem for a while now as well. But finally figured out a solution. While browsing the Unity forums I found that someone in the NativeFilePicker thread had this issue and linked another asset that solved his problem, an asset by the same author even and its also free:
https://forum.unity.com/threads/native-file-picker-for-android-ios-open-source.912890/page-3
What he suggests using instead is the Runtime Filebrowser or Simple File Browser:
https://github.com/yasirkula/UnitySimpleFileBrowser
The github page links the asset store as well where it can be downloaded for free also. I did not think this asset supported Android as well, but it does.
There is this issue to note though, it has been solved, but the Start method should be changed to Awake and you should add the code suggested here:
https://github.com/yasirkula/UnitySimpleFileBrowser/issues/87
But also you need to update the SimpleFileBrowserCanvas as it is not setup for VR. It can be found in "Assets\Plugins\SimpleFileBrowser". It needs to be changed to world space in its render mode and the scale should be adjusted. I changed it to 0.0048 in x, y and z. Experiment a bit.
Also give it a TrackedDeviceGraphicRaycaster so it can be interacted with in VR, and I am not sure if it helped but I found the disabled Eventsystem and updated its input system to something newer Unity has now that I suspect works better with VR and XR.
Not sure if maybe there is a way to always make this pop up in front of you because by default it has a specific world space position which is not very practical.
This simple file browser has the same problem as the native file picker. When you click browse and then select a folder from the native popup, it gets stuck when you exit the popup. The solution he proposes is basically reverting to the old file system. It will work on movie/image/sound files if you have read permissions for them, but not otherwise.
I can make the native browser work in a hacky sort of way if I press the oculus key after leaving the popup. It goes straight back to my app. It seems like the app is being left suspended. It seems like I should be able to call some of native android function to bring my app back to focus but haven't had any luck yet finding one. I found this link https://stackoverflow.com/questions/38452198/move-app-from-background-to-foreground-using-its-package-name-and-movetasktofro and tried to use the information there but so far it isn't working for me.
