Not likely...most mfgs and knifemakers heat treat and temper the entire blade and more often than not the handle also. If you're using a belt sander or grinder to sharpen you have to be very careful not to over heat the steel using a quench tub frequently. That is about all that would change the temper of a blade. If the blades were not heat treated or tempered properly at first..that could give you problems also.
I agree 100%.
That could also happen on a hamon if they have the clay too far down on the blade, or some makers like to only do an edge quench but normally the quench line is well defined.
Some newbies can over heat part of a blade doing a secondary bevel and sell it anyway.. if the blade has even turned a little blue it's got too hot and lost the temper.
God is good always; always, God is good!
"If thou wilst a keen edge win
forge it thick and grind it thin"