Hi Gerben,
When you achieve a breakthrough, the number that you saw on the Garmin in-ride are outdated. When that activity is uploaded to Xert, the entire activity is re-analyzed with your “new” signature, so by definition, MPA can only touch power one time along the activity. If you want to confirm what you saw on your Garmin, and see the difference between the “starting” fitness signature and the “new” signature, go to Advanced MPA --> and click ‘previous’ in the bottom right. This will show the analysis of the ride with the starting signature from the Garmin. Hope that helps! Cheers