I have the same issues. 2018 Kickr. Way too low resistance. I have to use 50/11 on “Zwift Island Climb” and with 90 in cadence I can only hit 310 watts before I spin out. My FTP is higher than that. Everything is calibrated/reset as described. Weirdly enough I need to use an easier gear on the downs afterwards. Very frustrating, and was dropped from a race yesterday due to not having enough resistance.
ATV4 via ethernet, BLE connections.
Reported here also https://forums.zwift.com/t/wahoo-kickr-resistance-too-low/13253/27