Because the power limit of charging is determined by the match of the phone and the charger. Although the rated power of the Xiaomi 80w wireless charge is 80w, if the phone used does not support the power, or the charger does not match the phone, the power will be limited. Therefore, even if the Xiaomi 80w wireless charger is used, it is necessary to ensure that the phone and the charger match in order to achieve the maximum power of charging. It is worth mentioning that wireless charging is slightly less efficient than wired charging, so charging speed may also be affected.
It may be that the phone only supports 50 watts of wireless fast charge.
Because maximum power can only be reached under certain circumstances. And after upgrading miui13, the maximum power is limited to 50w. That's why it shows 50w when charging.