i work for a packaging manufacturer

With PH stainless there are a couple of possible issues.
One is that when it is aged the size changes.
This is well known and documented and many designs account for this in the finishing.
If there is grinding done after aging then a stress relief is commonly used.
Often it uses a temp that is 25-50F below the aging temp.
This won't change properties and it will give good results.
Does your heater run full length?
Does the heater produce uniform temperature?
I am suspecting that the move to a lower thermal conductivity/higher thermal expansion alloy has changed the temperature profile.

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P.E. Metallurgy, consulting work welcomed

Looking at an individual crystal of some alloy at one end of the size scale and pretending a metal is completely homogeneous at the other end makes the reality that in between are crystals that are aligned and deformed by various processes so that any heat treat that causes the individual crystals to change shape in some direction can cause a non-homogeneous deformation of the item.

It is even worse if the crystal deformation isn't uniform through the thickness of a material - as is commonly the result of rolling. This can provide significant bend and twist embedded strains just begging for release. If identical amounts of material aren't removed from both sides - I've seen 0.250 inch thick aluminum plate pop like a Pringles chip when material was removed from one side.

Add in any potential for non-uniform thermal expansion due to non-uniform temperature or restraint and it's a miracle this sort of problem isn't more common.