@atanasuzunov Last time I checked with a powermenter a couple of weeks ago, steady charging on Zenfone 8 worked as intended. (Voltage and current dropped according to spec.)
While it won't give you any exact figures, SMMI test can at least show you if the values are lower when steady charging is enabled:
Open the calculator and input .12345+=
Single test -> AC charger test
Try it with and without steady charging enabled