can't update single profile field

Works as intended, you replaced the existing data.

If you want to add data select 'Add Data'.

If you want to only replace data in a particular field, you could delete the profile field, thus deleting existing data, then use 'Add Data'.

Test first!

* I meant delete the field, then recreate it, then Add Data.

This may be due to the way the Profile module works: it doesn't allow updating a single field, you have to retrieve all the field values into an array (for the relevant category if desired), edit the desired one, then save it again (either using profile_save_profile or user_save). Or you could make your own version of profile_save_profile that is more flexible.

This is a bug actually.
The function profile_user_import_after_save processes any defined profile fields by setting imported fields, resetttings any others.
A simple check solves this issue :

function profile_user_import_after_save($settings, $account, $password, $fields, $updated, $update_setting_per_module) {
// get all fields
$profile_fields = profile_get_fields();
$data = $old_data = unserialize($account->data);
foreach ($profile_fields as $field) {
+ if (isset($fields['profile'][$field->fid])){
profile_user_import_save_profile($field, $account->uid, $fields['profile'][$field->fid][0], $updated, $update_setting_per_module['profile'], $data);
+ }
}
// data column in the user table needs to be updated
if ($data != $old_data) {
db_query("UPDATE {users} SET data = '%s' WHERE uid = %d", serialize($data), $account->uid);
}
return;
}