why does "make delete-old" not delete some directories?

Hi,

Why does 'make delete-old' seemingly fail with some dirs and not others? 

context is earlier stable/13 updated to stable/13 as of a few minutes
ago.

root@redacted:/usr/src # yes | make delete-old

[...]
rmdir: /usr/lib/debug/usr/lib/i18n: Directory not empty
/usr/lib/debug/usr/lib/flua
/usr/lib/debug/usr/lib/engines
rmdir: /usr/lib/debug/usr/lib/clang/13.0.0/lib/freebsd: Directory not empty
rmdir: /usr/lib/debug/usr/lib/clang/13.0.0/lib: Directory not empty
rmdir: /usr/lib/debug/usr/lib/clang/13.0.0: Directory not empty
rmdir: /usr/lib/debug/usr/lib/clang: Directory not empty
rmdir: /usr/lib/debug/usr/lib: Directory not empty
/usr/lib/debug/usr/bin
rmdir: /usr/lib/debug/usr: Directory not empty
/usr/lib/debug/sbin
/usr/lib/debug/libexec
/usr/lib/debug/lib/nvmecontrol
/usr/lib/debug/lib/geom
rmdir: /usr/lib/debug/lib/casper: Directory not empty
rmdir: /usr/lib/debug/lib: Directory not empty
rmdir: /usr/lib/debug/boot: Directory not empty
/usr/lib/debug/bin
[...]

root@redacted:/usr/src # file /usr/lib/debug/usr/lib/clang/13.0.0/lib/freebsd
/usr/lib/debug/usr/lib/clang/13.0.0/lib/freebsd: directory

Shouldn't they be deleted?
-- 
J.