Ah thank you! I must have missed that (and in the documentation as well)!
Sorry to have bothered you.
Although libexiv2.so points to libexiv2.so.27 which in turn points to libexiv2.so.0.27.2, the new version is libexiv2.so.0.27.6 and the names in it (including arguments) are not compatible with the system one. That is unfortunate.
As I can see all the metadata using exiv2 (including the embedded previews) I was under the impression that rapid would simply call exiv2. I am not familiar with meson etc (I only see “Run-time dependency exiv2 found: YES 0.27.2”) so I am going to mess around with the system further.