This is not always, but often.
I just tried to post 2 Amazon links...with spacing and more copy between them...and only one appeared.
As said, this doesn't happen all the time, but is annoying when it does.
Ah, if I follow the links in my 11:07 post, there is "/?tag=theans-21" appended to the end of the URLs I pasted. I guess this is added by AB to indicate to Amazon that the referral came from AB's website.
I therefore reckon that, if AB detects a link to 'amazon.com' or 'amazon.co.uk' in a post they append the tag to the URL and use "Amazon.com [or Amazon.co.uk] User Recommendation" for the link text. That doesn't explain the failure to post 2 links though, and I don't get that using MS Edge browser.