You've exactly described what happens on upshifts. There'll be a bit of slack created in the top loop as the transition zone comes over the top of the cassette and the pedals will slip forward a bit.

Most riders don't notice it because they lighten the pedal load a bit during shifts, and have come to expect it and so tune it out as a normal part of the shift process. Better systems also do this, but to a slightly lesser degree because the transition zone is better managed by means of shift gates and phasing of the sprockets.