Watt and rpm issue

Hi, I purchased Zwift Hub in July and COG a few weeks ago. Today, as I finished a stage, I noticed an “up and down” in watts and rpm, coinciding with the most significant gradients of the climb. I tried to keep power and pedaling speed constant as much as possible but the problem continued, even if I decreased power and/or rpm. I noticed that this problem occurred together with a sudden “hardening” even though the slope was constant. Once the climb was finished, the watts and rpm returned to normal. I also made a video.