profile does not write to user->data

I have a small number of users whose profile fields have been serialised into user->data. they wwre all created within a narrow timeframe (older and newer users don't) so i suspect one version of profile.module serialised and now does not.
At line 528 of my version

function profile_save_profile(&$edit, &$user, $category, $register = FALSE) {
 ...
    // Mark field as handled (prevents saving to user->data).
    $edit[$field->name] = NULL;

I have a theme function foo_username($user) (overriding theme_username in includes/theme.inc).
The $user passed to this function has $user->profile_... populated ONLY for those users where the profile field has been serialised into data.

Why has profile stopped serialising into user->data???