The algorithm on Stryd processes and filters the data measured by the sensors. The processing of the data takes a few seconds. Under normal conditions, this is barely noticeable but when running short intervals using the lap function on your watch, you may notice that the average power for the lap is a bit lower than expected. This is normal at this time.
The workaround is to start the interval first and press the lap button on the watch after about 5 seconds after starting running harder.