Charging and thermal suggested change
1. (Further understanding of QC3.0 protocol, contents of old Post may not be valid) Took that power bank apart and looked up the chip number. The chip was little buggy the manufacturer admits, as it outputs 7.3V upon entering QC3.0 continuous mode. It is still in the specs though. I can charge other device because those device observe the voltage and request higher / lower voltage accordingly. It seems like Zenfone 6 does not observe the voltage given by the source supply and requests higher voltage rather blindly (requests 20 times +0.2V from 5V to 9V). Try and change that. This can have extended benefits as some cables which have high resistance lose some voltage, and observing the input voltage at the phones basically compensates for that.
2. The phone cranks down the charging current if CPU/GPU working and heat dissipation is not enough. Perhaps allow the user to set charging to take priority in lighter processor loads, so that the CPU and GPU clocks gets cranked down instead, maintaining the fast charging.
If you don't understand what is happening from my observations, you can comment below