App can't see its obb file

Hey folks ... looking for any insight here ... we are moving from 4.27 to 5.2 ... everything builds and packages fine through editor ... installing the app and everything shows up correctly under unknown sources, but running the app just gives the infinite three dots.

SDK and NDK are android-32, and we can launch and run successfully via the project launcher in Editor.

Running adb logcat it looks like the system can't actually find the obb when it tries to execute, and goes into a download wait, which is the infinite three dots. Even though the obb is definitely there on the headset under Android\obb ... 

Added some prints to the default logging and the path it is using to call File.Exists() (in java) is correct and browsing with ADB shell to that location show the file is there, but file exists just fails  -- We grant internal/external read and write permissions.   

Kinda at a loss as to what the hell is going on ... and why it isn't able to locate the obb. I can't try packaging the data within the apk as it's over 2Gb and the build fails ...

Any help would be much appreciated ...

[copy  + edit of Richard's post on Unreal dev forum]