For avatar gallery subfolders like images/avatars/gallery/cat/cat the path will be treated as cat%2Fcat which triggers HTTP 404 errors on avatar images located in images/avatars/gallery/cat/cat subfolder.
This is due to the path handling (/ to %2F replacement)
https://github.com/phpbb/phpbb/blob/3.1.x/phpBB/phpbb/avatar/driver/local.php#L185

rawurlencode($cat)