Like Dertosh, I came across bUseExternalFilesDir in my travels to realise it instead means bUseInternalFilesDir in Distribution Shipping builds only.
So if you’re running BuildCookRun, be sure to build for Shipping and add the -distribution parameter to the end of your build command (or enable “For Distribution” in the packaging window), as the MakeApk code in UEDeployAndroid.cs will set bUseExternalFilesDir to false if not building in distribution mode.
Additionally, if like me, you decided to put an inline comment after the property in the config file like so:
bUseExternalFilesDir=True ; Need this for Save Game
Remove the comment, as apparently the UEDeployAndroid.UseExternalFilesDir method was still returning false for me due to it.
Note: Leaving this here for any folks coming from search engines. I had to enable this option to get my Save Games working in when WRITE_EXTERNAL_STORAGE needs to be disabled (a requirement of Meta/Oculus Quest’s VRC).