My interpretation here is that I'm thinking there's a problem with the I2C bus used for communicating with the battery.
I pulled some schematics of the iPhone XS Max, and the BMS communicates over I2C0_SMC. That signal goes to a chip labelled USB-PD and another one labelled Yangtze Charger. It looks like there's a level shifter circuit right next to the battery and of course caps and resistors in the line.
Although I have reference designators for those components, I don't have a board file that shows me where they're physically located, at least not yet.
However, the other thing of interest is that those signals appear to go through the interposer, meaning they're transferring from the top board to the bottom board (or vice versa). So in addition to the possibility of a missing or damaged component, there's also the chance that the interposer has flexed and broken some pads. That, of course, will take expert-level microsoldering repair, unfortunately if that's the problem.