The reason for this is Facebook's cache system.
Whenever you put a new link into Facebook, it takes a while for it to figure out where that link is going. Once it does, it writes that into memory so it doesn't have to do it every time.
Therein lies the rub.
If you originally test your link before you have an ASIN entered in ReaderLinks, it'll send the person to your fallback URL (usually your author home page). When that happens, though, Facebook says, "Ohhh...okay, so this link resolves to yourhomepage-dot-com. From now on, we'll just show that."
You may also have been on a VPN or have asked a friend or PA from another country to test your link before you did. Wherever they're located will be the end result. So, if you want your links to resolve to Amazon US, you have to first make sure Facebook has learned it in Amazon US.
Right, so how do you fix it?
It's actually pretty easy.
Step 1: Get the ReaderLinks (or whatever) link you want to train Facebook on.
Step 3: Enter the URL in the top line and click the Debug button.
Step 4: Look at where Facebook is thinking it should go.
Step 5: Click the Scrape Again button and force it to relearn based on your browser and the location now being Amazon, Audible, etc.
That's all we do when we're asked to help with this. Do note that sometimes you have to click the Scrape Again multiple times to get it to catch on.