I just started seeing this again as well. I saw it first back in July and there was some discussion then about the use of hard-coded vs random user-agents (
https://www.mobileread.com/forums/sho...d.php?t=276443). But I installed the 2.75 Calibre update a few days ago and noticed this behavior again yesterday. The trick of changing the ID tag from mobi-asin:XXXX to amazon:XXXX seems to fix it, but I thought I'd bring up the user-agent issue and ask if that might also be involved.