Keep in mind that GOG always displays the messages from the "online" members first, which is quite annoying... That's probably why you saw that old conversation...

All my private messages have been successfully transferred to the new chat system, and all the messages from a given member are in chronological order. I always get the most recent one when I select a member.

The member list on the left side is, however, shuffled and unpractical, and no search function has been implemented yet. You need a flashlight and a shovel to find some members!  That chat system is still beta and we're the guinea pigs!  Let's hope that GOG is listening to the users...  :-|
 
 
 
I'll start three giveaways soon, and the new chat system is highly irritating... Here are three advices that might be helpful for your upcoming giveaway  (regarding the chat):
 
1)  Write your private messages in Notepad, Notepad++, or any other text editors, and when you're done, paste them in the chat's textbox. This way, you'll be able to see the messages as a whole before you send them. Two lines are not very practical for editing messages!

2)  Never use the Enter key to create a new line, even in a text editor. Instead, use Shift+Enter, otherwise you'll create partial messages. The Enter key is used to submit messages...

3) Ask all the winners to post in your giveaway thread when they receive your private messages, just in case...