Agreed. OPPO20w charging head can give the Mi 8 fast charge. The charging sequence must be correct. When you use the bank to charge, is there any spark at the moment when the charger head is plugged in? It's a brief wave of current and voltage called a surge. The damage to the equipment is very large.
If you use the original charger, first even the power supply or even the phone has no effect, you are happy; However, if it is not the original charger, the first connection to the phone and then the power may produce a relatively large voltage pulse, which will bring harm to the phone.