I think that the biggest loss of temp, is due to the portafilter starting temp, moreover it seems to me that the preinfusion is almost 10 seconds and i hear the motor began to push down the piston at 15s and the pressure began to rise 7s later.
Anyway, in a subsequent video, the tester, he had an issue with a faulty temp probe, so maybe must to wait the next videos once the temp probe will be replaced, to see if something change.