Found !
The culprit is the plugin "Amazon.com Multiple Countries".
I think that if you install it you'll be able to reproduce the bug with a book having amazon_fr instead of mobi-asin in its metadata.
But there is a thing that I don't understand : the plugin (it is the only one that I have) was visible even having launched Calibre with the arguments to not load the plugins!
Here is exactly my command line:
- "C:\Program Files\Calibre2\calibre.exe" --ignore-plugins
And even with this command line the plugin is visible in the list of "Installed plugins". That's weird.
A third thing : the foldername keeps on having the name "Calibre2" albeit the installed version is now the version 3 ? For compatibility in case of upgrades, perhaps...