Strange behavior during charging

Hello fellow ebike enthusiasts,
A question .. hopefully someone has an idea what is happening? When I'm bulk charging my pack, (lifepo4 15s prismatic) the first cell in series dips to 0V red, and back to 3.2X, then back and forth, while flashing that it is balancing I think as well. (I'm monitoring with the Bluetooth bms but directly charging and discharging) Any ideas? Hopefully it is just a monitoring issue, the cell otherwise appears fine physically and reports to be ok besides this off behavior.
Thanks

I'm assuming maybe the bmsneedsto pullvoltagefrom cell#1 during the balancing process, I'm hoping..