This is an interesting theory. I just tried again on my discourse installation and the 1st link I tried turned into a onebox just fine but then I tried 4 more unique links (maybe 10 seconds apart) and those all resulted in a 404. I waited like 15 minutes and tried another link and got another 404.
I’m a little unclear on whether and how often past oneboxes reach back out to Amazon for info. Anyone know off the top of their heads?
It would good info to have for this mysterious situation plus for compliance with Amazon affiliate terms because I think if you are going to pull and display certain pieces of information from Amazon, you are supposed to update it every 24 hours to keep it up to date.