The address in question is actually 32bit. If you write 32bit value of FFFFFFFF to that address it won't hurt you, except for the fact that you're still gonna be playing in a few weeks because you just set your number of plays to 4,294,967,295.
That's not trying to object to your valid concern, just a note that in this case it definitely doesn't break anything. I agree with you, using a 32bit write here when, reasonably, we only need 8bits (in a sane environment the three other 24 other bits will always be 0 anyways) is overkill. But then again, you could argue that you want to be 100% sure they will be 0, so you set them regardless using a 32bit write. One can argue both ways here.