$new_pass always returns null – password_reset hook

Note that:

add_action( 'password_reset', 'remote_password_update' );

is like calling:

add_action( 'password_reset', 'remote_password_update', 10, 1 );

where 10 is the default priority and 1 is the number of arguments.

You need:

add_action( 'password_reset', 'remote_password_update', 10, 2 );

with 2 as the number of arguments. Now you should be able to access the $new_pass input variable.