The weird reading comes from the missing lead messing up the way the BMS reads voltage. You could just solder that one to the reatbof the group, but i recomend soldering all of them together at the back of the BMS and removing all but one of those leads that go to your group 16.
After that you should be able to plug everything in and it will read lile it should. If it doesnt you will need to go into the settings menu and charge the BMS from 21s to 18s