In login/lib.php has an IF that checks if it's called home.

if ($urltogo == ($CFG->wwwroot . '/')) {

Within this IF there is another one that validates several other options

if ($urltogo == $CFG->wwwroot or $urltogo == $CFG->wwwroot.'/' or $urltogo == $CFG->wwwroot.'/index.php') {

but it only arrives at this IF with the clause "$urltogo == $CFG->wwwroot.'/'"

and never with "$urltogo == $CFG->wwwroot" and "$urltogo == $CFG->wwwroot.'/index.php'"