This does not ring any bells for me, and I see nothing in the dune changelog that would obviously be relevant.
That will be seen by a more relevant set of people, so perhaps they will just know.
I myself am not very familiar with the way dune looks for installed libraries, but from looking at the code, it seems to search for them in the [findlib_path] directory, so it looks for the findlib package in [/usr/lib/findlib].
Do you have an easy way to check that that directory exists on the filesystem and what files are in it?