Editing comments trigger authorship changes

Hello,
I am experiencing the following bug. At the beginning I thought it was because of some weird hook I installed, but I debugged my code and I could not find any bug from my side. What I experience is the following:

When the Administrator user edits a comment, everything works fine. When an user (with permissions to do so) changes a comment that NOT belong to him, drupal changes authorship to him, instead of keeping the former creator. I put a print_r in comment_save, before drupal could perform any save onto the DB and this is what I got

stdClass Object
(
    [cid] => 606
    [pid] => 0
    [nid] => 26762
    [uid] => 97
    [subject] => (Nessun oggetto)
    [hostname] => x.x.x.x
    [created] => 1450696779
    [changed] => 1450696778
    [status] => 1
    [thread] => 04/
    [name] => x
    [mail] => 
    [homepage] => 
    [language] => und
    [node_type] => comment_node_thread
    [registered_name] => x
    [u_uid] => 99
    [signature] => 
    [signature_format] => plain_text

As you can see, uid and u_uid DO NOT match. uid is the user id of the user (me in this case) that is changing comment id 606, whereas u_uid 99 is the former user.
Is anyone experiencing this issue? Is there a way to work this around?