If I may interfere here?
MarshMellow introduced important features on Android. Seeing the external memory (the SDcard) as internal is one of them.

Android 6.0 never will be officially released for my smartphone, not even 5.0 will. So far I am on 4.4 (rooted) and have no issues.
Anyway 6.0 is in alpha on XDA, I don't intend to update soon, but I do follow the progress there. I read that many users have problems with the memory usage as this gives unexpected consequences with some apps. And the choice of using th SD as internal or not is made just after flashing and irreversible unless : Users have already reflashed the Android and reformatted the SD to fit their intentions.
I don't know if the G4 has the same issues (even if Marshmellow is LG-made), I am just saying. However it worked before the update to 6.0, right?