I started Calibre in Debug mode and did all the same steps...
Images here:
https://imgur.com/a/2idmfkK
Grabbed a purchased book from Amazon, saved it to downloads folder, Calibre auto added/converted. (images 1 & 2)
Then I emailed it to my Kindle. (image 3)
Proof it was converted and emailed. (images 4, 5, 6)
Amazon's failure email. (image 7)
Debug .txt file is attached (I redacted only my last name with xxxxxx for privacy).
After re-opening Calibre, I did check to see what Input Format defaulted and it was incorrectly AZW3 (as expected) for this newly added book. (image8). But West with Giraffes (the one I individually converted) still defaults to MOBI, and Golden Poppies (which I did
not individually convert) defaults to AZW3.
Let me know if there's any other info you need to get this bug fixed! Thanks!