For those who could have the same issue, I just solved it by following some tips from Trainerroad, in essence: “maintain a mid-ring gearing in the rear cassette, and either a mid (if you have three) or inner (if you have two) front chainring. Your chain should follow a straight path from rear to front. This will also help you avoid hitting a wattage floor.”
Maintain your gearing - In most cases, you should not change gears once you’ve set your gearing. Changing your gears may confuse your smart trainer and can result in a temporarily uneven delivery of resistance.
Cadence can directly change your output power during your workout if you’re changing your RPMs significantly (e.g. >10-15 RPM). When increasing your cadence, your power will attempt to rise; to counteract this, your trainer will lower the resistance. The opposite will occur when lowering your cadence; an increase in resistance will be felt.
Anticipate upcoming intervals by slowly ramping your cadence before the interval begins. You should ramp to your desired cadence that you’d like to maintain during the interval. This will help you be able to manage the upcoming change in prescribed power.
Maintain a steady cadence - With the exception of the previous point, it’s recommended to maintain a constant cadence within a range from 80-100 rpm during your workout. This is the best practice unless otherwise stated in the workout description and/or text.