This post was very helpful. I have the X-T3 and have been seeing the same grip issues. It seems there is either a short or faulty contact between the grip and the camera. The display sometimes shows all three batteries, then switches to just one battery (the in-camera). I also saw a brief warning displayed about the camera grip draining the battery quickly but I couldn't read it completely before it disappeared. I'm guessing the camera thinks I'm using the internal battery to power the main body and the grip; and it is not recognizing the two batteries in the grip.
It's disappointing to know this has been a problem going back to the X-T2 and it hasn't been completely fixed or addressed with the X-T3 grip. Now that the X-T4 is out I wonder if it will also have the same challenge.