please dont leave this thread, I have exactly the same problem that you have (running MX-Linux 19.2, all lensfun 0.3.2-4 packages via Synaptic Package Manager). I see this problem with Nikon D850 + Nikko5 105mm 1.4 ED (which is NOT supported by lens fun). Something is not right.
I have found that lens-fun provides very few lenses compared to LR, particularly for Zeiss lenses…But, hey, this is open source world, so I will try to understand how to contribute to add my lenses to lensfun (does not seems obvious at first glance). Stay tuned