SUMMARY

kffilemetadata is looking for external plugins at the path:
    /usr//usr/libexec/kfilemetadata/externalextractors

which is clearly wrong. In the source code, this path is defined in src/config-kfilemetadata.h.in:
    #define LIBEXEC_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/${KDE_INSTALL_LIBEXECDIR_KF}"

Unless there's something weird Gentoo is doing, `CMAKE_INSTALL_PREFIX` is resolving to `/usr/` and `KDE_INSTALL_LIBEXECDIR_KF` is resolving to `/usr/libexec/kf6`. The net effect is that `LIBEXEC_INSTALL_DIR` is being assigned `/usr//usr/libexec/kf6`.

This used to work with kf5, but something must have changed when transitioning to kf6.

STEPS TO REPRODUCE
1. Create an external extractor and store it in /usr/libexec/kf6/kfilemetadata/externalextractors
2. In dolphin, open the properties of a file relevant to the externalextractor and view details
or
1. strings -e l /usr/lib64/libKF6FileMetaData.so.6.4.0 | grep libexec

OBSERVED RESULT
No metadata is shown.

and

$ strings -e l /usr/lib64/libKF6FileMetaData.so.6.4.0 | grep libexec
/usr//usr/libexec/kf6/kfilemetadata/externalextractors
/usr//usr/libexec/kf6

EXPECTED RESULT
Metadata is shown.

and

$ strings -e l /usr/lib64/libKF6FileMetaData.so.6.4.0 | grep libexec
/usr/libexec/kf6/kfilemetadata/externalextractors
/usr/libexec/kf6

SOFTWARE/OS VERSIONS
Linux:  Gentoo Linux 2.15
KDE Plasma Version: 6.1.4
KDE Frameworks Version: 6.4.0
Qt Version: 6.7.2

ADDITIONAL INFORMATION

I've patched my local build of kfilemetadata by simply removing `{CMAKE_INSTALL_PREFIX}/` from the `LIBEXEC_INSTALL_DIR` definition, and that fixes the issue for me.