DIRECTORIES is more needed on Windows than on Linux and Mac because it has no equivalent to RPATH. Linux rarely needs DIRECTORIES (and warns you if it finds something in a DIRECTORIES argument), and on Mac it isn’t even used at all.
We deliberately made the decision to not consider the PATH variable in file(GET_RUNTIME_DEPENDENCIES), because setting the PATH from outside the project could change the results of it. You can add $<TARGET_DIR:qt6core> to the DIRECTORIES argument, which should allow it to find it.