First, this situation should not be happening. I have not used S10+ but I have previously found old phones that have faulty proximity sensor that caused this. So, it might be the case for you.
I would definitely agreed that this is most likely due to the battery. This is an iPhone 6, if you haven’t replaced the battery yet, it is time to do it now.
You guys sound truly excited… So am I while reading this. A big win for the repair community and consumers as a whole!