Something is definitely not right. Those displayed temps on a wet coil with barely a few watts says to me resistance is rising with temp more than the board expects. To try to rule out another variable, set the TCR in"special" to 0.004 in one of the profiles, upload the settings and then select that profile to use. Run the same tests and see if the results change. If not, it could be the intermediate connection for the positive pin in the subtank.
I try anything as soon as I can. I've seen the same behavior with prebuilt Nickle Kanger coils. The only real wattage adjustment I get is of course in Kanthal Power mode.