`HELPER_DIR` incorrectly substituted in `*.desktop` files when using Meson

For example, doing:

meson setup ${srcdir} -Dwayland=disabled
meson compile
meson install --destdir=/tmp/z

results in helpers being installed to /tmp/z/usr/local/lib64/xfce4. However, the installed .desktop files use a path without xfce4 subdirectory, e.g.:

X-XFCE-CommandsWithParameter=/usr/local/lib64/xfce4-compose-mail mozilla-remote %B "mailto:%s";/usr/local/lib64/xfce4-compose-mail mozilla %B "mailto:%s";

If I override -Dhelper-path-prefix=/usr/local/lib64/xfce4, then the .desktop files have "correct" path now, but helpers end up being installed in /usr/local/lib64/xfce4/xfce4.