What does it mean this strange behaviour of the power line in activity?
That means the power has been clipped because Intervals.icu thinks it is too high relative to your FTP. Lots of power meters generate spikes and it is important to filter them out.
Activities with spikes get an orange bolt on the calendar:
And a note under the chart that you can click to adjust the threshold. You might also want to look at your FTP. It may be too low:
You can click “Charts” and choose “Raw power” to see power without any spike fixing.