As i understand it. U planned say FL330. Crack at FL210. Level off at FL250. QRH procedure. U remain in FL250. Cpt resets FL ALT indicator. Thatīs very very reasonable. Cause due to ur level off the deltaP rose further (since the auto cruise mode was not yet reached). Adjusting the FL ALT assures the correct deltaP again.
However if u climbed further again and then resetted the FLT ALT to a lower level, thatīs no good. See previous comments.