Actual power in erg mode workouts settles below target power

Hi all,

There is a strange fenomenon when I do a Zwift workout in ERG mode. In the work sections the actual power always settles a few watts below the target power. After increasing cadence power again settles below target. During warmup & cooldown phases the power is spot-on. The bike itself is in a fairly light gear somewhere like 36-19. Trainer difficulty is set at 75%.
My setup is a new Kickr Core (firmware version 1.0.13) talking to a Win10 laptop with an Ant+ dongle. Not using the companion app. Recalibration has no effect on this. I am using Garmin cadence sensor and heart rate monitor during the sessions. Otherwise the whole thing works like a charm.

Any idea what’s going on here or where to look?

Kind regards, Jan van Mastbergen