This is pretty common really, and isn't necessarily bad heat treatment. Often when steel is quenched it will distort if there's any residual stress. The razor makers of course need the spine to be reasonably straight, so they straighten the razors as close as possible then grind them. Over time, the steel will tend toward relaxing back to the position of lowest residual stress, so if the original bend was significant but was straightened, it can come back gradually over several years.

One way to almost completely eliminate this issue is to perform a stress relief thermal treatment before heat treating and then do not straighten if the steel goes out of whack, but rather grind it until you get where you need to be dimensionally and straightness wise, but I don't think many would ever do this as it adds a good amount of time and therefore cost to the finished product.

So instead they go the route that they do, and now and then you get a stinker... but you almost never know that when the razor is new so it doesn't hurt sales.