installing tq 11 the overwrite option does not work

thinkpad t400 librebooted
trisquel 11 iso on an usb memory stick
sata hdd 500gb

During installation a for more security, overwrite empty
disk space option displays. I
select overwrite but the overwrite does not take place. Trisquel 11 gets
installed in about 20 minutes. Therefore
overwrite has been skipped. Have you noticed this
error? Apart from skipping the overwrite I have not
noticed errors testing trisquel 11 when installed.

I have repeated installation of trisquel 11 from several
usb memory sticks. Every time overwrite gets
skipped.

On debian if you select the overwrite option, a balk displays,
which shows how erasing the hdd moves on. Thank you.