I would think that an improved release would result in more force applied to the arrow, and thus would tend to make a given arrow leave the bow less stiff than it would with a poor release.
So what could cause the opposite?
--If your draw length decreased at the same time you changed your release.
--if you were getting a false weak before, because the arrow was recoiling off the strike plate, and your changed release added enough force to eliminate the recoil and thus is now showing a true stiff shaft.
--You could have been rolling the string off your fingers more with the poor release, thus flipping the nock to the left.
There could be other reasons I can't think of. Probably the best thing to do is to re-tune your arrow so it is flying true, given the changes in your form. I've had to do this several times over the years because of form changes. Something as simple as changing the weight distribution among the fingers on the string will affect tuning of the arrow.