Anders_ASUSOk. But the chargers I use aren't cheap. They all are from reputed brands. They all are the OEM chargers that came with the phones I brought like Oneplus, Samsung, Motorola, Xiaomi, Gionee, Oppo and Vivo. Still not a single one is able to charge the 6Z at 10W. However, the same ones are able to charge other smartphones with similar battery specifications and capacity at 10W. . This is so frustrating.
This is most probably not a bug. Rather a question about if there was a proper handshake between the charger and phone. What I'm saying in plain English is that they are not compatible. This is why your phone defaults to only 5W. It's a security thing. If the charger is a cheap Chinese charger that doesn't support the right charging protocol, this is what happens to prevent any accident from happening.