koala logs

You actually output more power in the flats and lose significant amount of power on climbs. It can be caused by few things. Not pacing your effort, too low heart rate for the climb (insufficiently warmed up), and bike fit. A bike fit that is "super" comfortable for cruising in the flats tend to work less well on climbs. The typical adjustment for climbs is moving the saddle forward and maybe tilting the saddle a little bit downward, nose down and everything else the same.

You'll need a different "warm up" for climbs. You wrote you feel effortless on the flats. That's not good for climbs. You'll need to put a bit of effort before a climb. Enough to make you breathe harder but not too hard you start feeling the burn on your legs. Hold that effort for a few minutes before a climb, move closer to the front of the group to get less draft and work your legs harder.