can't add .AZW3F files to Calibre

After upgrading to a new Kindle a couple of years ago and having a major headache trying to reload my previously purchased ebooks from Amazon, I now save a copy in Calibre on my PC. Purchased a new ebook from Amazon last week but can't add it to Calibre.

Seems Kindle/Amazon are starting to send me AZW3F files in-lieu of AZW3
and Calibre rejects AZW3F files
Have never had any problems with AZW3

Anything I can do, besides stop buying from Amazon ?

Thanks