src.conf: setting WITHOUT_DEBUG_FILES has no effect

Setting

WITHOUT_DEBUG_FILES=YES

in /etc/src.conf is supposed to prevent building debug files for shared
libs and binaries - that is my interpretation of the manpage of
src.conf(5).

After each run of 

buildworld buildkernel && installkernel && installworld

and performing a

make delete-old

the cleaning skript picks up lots of files every time after a rebuild
and installation of world situated in 

/usr/lib/debug/usr/tests/XXX

Is this hardcoded? Or do I miss here some extra knob? Or is it a bug?

Thanks in advance,

Oliver