I worked for a plastics IM company, there are a lot of factors that work for and against you forming plastic parts like this - temperatures, lot to lot plastic variation, pressures, mold temperatures, gate sizes, gate design, gate wear, ring heaters failing, and on and on. Just looking at that lower I can imagine that some minor pressure / temperature would cause that end of the lower, the part without the metal substrate, to deform as it cools or is staged after being formed and perhaps fixtured for assemble while still hot.
These parts are molded with tons of pressure and [likely] 400+ degrees of temperature in some parts of the plastic flow process. They take a 'set' inside the mold, then are released probably warm. That one could have fallen into a corner for a moment, or maybe it was a fan used in the summer to cool the workers, or any number of other variables. Using a hair dryer could change the 'set' but unless you fixture it properly as you heat it to the correct temperature (whatever that is) you could cause a worse condition.
I'd send it back and let S&W deal with it. I am curious what their answer is - perhaps they have a fixture that they can fit the lower in and heat it up and re-"set" it.
My 45FS has a bit of a bend to it, you have to look for it to see it, but it seems to function just fine .
