FYI we traced this issue to an alternate source FET in the charging circuit. When the input voltage and battery voltage are very close to each other, the charger will operate in buck boost mode, and when this happens with the alternate source FET, the charger cannot fully switch the high current FET on and off, which causes the low input current that is observed.
If you are running into this issue, use a 20V charger, or a 12V charger. We recommend a 20V 60W+ charger to get the best performance from your system.
Supporting as wide an input range as possible is mostly to allow charging the laptop in situations where you may not have the correct charger but need to get some extra life out of the system.
So far not many people seem to be impacted by this. But if there is a large impact we could disable negociating 15V.