I've done a bit more testing using clean systems, the change happened between version 3.36 and 3.37.
Two screen captures are included in the zip that show the results.
For each version, a clean Windows 10 virtual machine was used, a C:\Calibre\Testing was created, Calibre 64bit was installed using the defaults (except for the library path)
The only setting changes was to turn on the "Allow un-authenticated local connections to make changes" option, and start the server.
Then the command line was run:
Code:
"\Program Files\Calibre2\calibredb.exe" --with-library=http://localhost:8080/#Testing add --one-book-per-directory "c:\Calibre\cdbtest"
Version 3.36 and earlier add the cover correctly, version 3.37 and on do not.
Interestingly if I do a set_metadata call with the id returned by the add call followed by the opf filename, the cover then gets set correctly, so the issue is just in the add portion.