Run it directly on your config file as:
SYSTEMD_LOG_LEVEL=debug systemd-tmpfiles --remove --clean /path/to/tmpfiles.d/foobar.conf
(I'd rather place custom configs in /etc/tmpfiles.d and leave /usr for the package management.)
Most likely the issue is that systemd-tmpfiles also considers not just the mtime but also access time (it wouldn't do any good if it started removing actively-used files just because they haven't been modified), as well as btime (birth) and ctime (inode change) – the latter two you cannot backdate with `touch`.
If you want to only consider modification time, specify it as something like "mM:10d" – see "Age" in tmpfiles.d(5).
Mantas Mikulėnas
