The file_creation_failure() function added to sanity-hsm calls df for debugging purposes but does not redirect the output of df to stderr. Since file_creation_failure is used in functions like make_large_for_cancel which are in turn used in command substitution

local f=$(make_large_for_cancel ...)

the df output goes into $f.

Also many of these functions check for success by calling path2fid which pipes lfs to tr

path2fid() {
        $LFS path2fid $1 | tr -d '[]'
}

Since tr will succeed this will fail to detect errors.