Darktable depends on exiv2, not lensfun, for lens name mappings. You may not have this lens mapping if it's an older version of exiv2. To compound the problem, exiv2 faces some difficulties when it comes to accurately providing this information, so even recent versions may exhibit this problem for some lenses. If updating to exiv2 0.26 or later doesn't solve ...
So, what works for me is:
yaourt -S lensfun-git -> uninstalls lensfun (non-git in the process).
sudo ln -s liblensfun.so.2 liblensfun.so.1
I do get a connection refused error when runnin lensfun-update-data, but seeing that the updates and infrequent, and that my lenses are all in Adobe's profiles, I'm happy with this.
For Adobe profiles, you need DB version 2. The differences are small, however: Besides the Adobe profiles, the <real-focal-length> tag became an attribute of <distortion>.
Darktable won’t care which DB version you use, In particular, Adobe profiles will work with any Darktable version, as along is it supports the API of current Lensfun.
The lens type in Exif may not be mapped to a usable lens name by the software you're using. In the case of darktable, this depends on exiv2, which may not have this lens mapping if it's an older version of exiv2. To compound the problem, exiv2 may not continue to provide this functionality in the future, so even recent versions may exhibit this problem for ...