Let's see to fix that then,
Disable PIA
OPEN ./ext/threedi/pia/lite/pia.php
Should be line 287 - function pia_main() - FIND and DELETE this line.
AND user_avatar_type ' . $this->db->sql_like_expression('') . '
SAVE and close that file.
Enable PIA.
Automatically at the first hit to the board the logic will (should)
populate all of the remaining avatarless users in one shot..
Pls report.
Edit: you should then hit submit in ACP to let it fill out those users.
The main logic called on a per board's hit only runs if there are logged-in new avatarless users.
The ACP instead forces that logic to run with no conditions, just the submit button.