Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

Was the bike itself at ambient temperature when you zeroed it? If it’d come out of the car the cranks and so forth may have still been warmer than when it reached the roundabout

This might be it, and thinking about it, I haven't raced on this bike since Sept-19, and 'MagicZero' was in the version 10 (Jul-19) firmware. I didn't update until Oct-20, and all sessions since update have been on the turbo at ambient temps.

Whichever way round it was, it’s annoying. Could you apply a correction factor to the pre-roundabout power, so your records are accurate?

Good question, @Greenbank might know - Does a Garmin store the zero-offset number during an activity?