lensfun updates work without any issues. (~/.local/share/lensfun/updates, and via sudo also in /var/lib/lensfun-updates/version_1.)
Might be an issue in snap due to sandboxing?
But for my own compiled darktable 3.0.2 from git there is no issue.
What I still face is related to building the manuals, and here I am still stuck with the errors I posted earlier in the threat.