@Rahul-Balaggan The third party uploading agency also seems to be under the impression that the automatic update system should work. Unfortunately, it doesn't since those directions are the first thing I tried.
And I understand, thanks for trying. I'll poke at Amazon CS for the update. I'll ask them out of curiosity if they know why the automatic update isn't available.
Edit: That was naive of me. Of course they told me to clear my cache. My money is still on my "some kind of anti-tampering mechanic was triggered from replacing too much of the text at once" theory. Oh well, I have an updated version now, even on read.amazon.com and even without clearing my cache. Simply pressing the refresh button once (not even a hard refresh) on the tab I was looking at it on brought up the right book after my chat with CS, where they said they "pushed an updated version to my devices".