evnchn1. (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.
This power bank does not obey Quick Charge standards and outputs 12V, knowing that this phone only have 9V input, you know the consequences. Once killed my phone, luckily it was within 10 days of purchase and I was able to return it after it self-recovered for 2 days and refuse to fast charge anymore. Using my phone I can verify it bugs out all the time. If you have this power bank, stop using IMMEDIATELY / only use slow charge port.
Meanwhile, ASUS, can you do some software safety to guard the device from problematic chargers? Maybe you can stop the the charging when voltage requested by QC does not match power bank real output voltage and does not trigger hardware safety.
Warning, Killer power bank watch out, awaiting fix