Recently I got a new comment to the postcard I received.
But the “comment” was actually a copy of MY Hurray message with “REPLY:” prepended.
That means, the sender copied the text and posted as a comment.
I felt puzzled for I didn’t expect it to be published, but it was just an ordinary “thank you” message and causes no harm when published.
Does anyone have similar experience?
I only copy mine and keep it in a private database I created in Notion
I guess they want to save the data and it’s easier for them to track this way, but I still think it’s better to at least inform the original writer if they’re comfortable with that before posting 
A Hurray message is private. I would feel uncomfortable if it was published as a comment. I would contact the admins.
You can delete all comments on your received postcards
Same as @GrauBunt . It’s making a private message public. That doesn’t feel okay for me and I would aks admins to delete it - or maybe first contact the Postcrosser.
Please alert the Postcrossing team by filling in the Contact Form (link is below) :
Choose “Option 3” > “Postcard Walls” > “Other” > Fill in the Form > Attach an Image > Click “Send Message”
If you didn’t hear from the Postcrossing team after a few days, you can re-send a new Contact Form.
Hope this helps!
+++++ Edited to add +++++
The “Comment” section for Postcard Walls is not allowed to post any kind of private messages. It is breaking the rules of Postcrossing. Admin would removed the said messages and send a warning to the sender involved. Please contact the Postcrossing team as soon as possible.
Thank you for message everyone!
I agree with “Hurray message is private”.
I would consider a little more before deciding what to do next: just delete the message, inform to admin or both.
Aha! Thank you! I did not know that the comment can be deleted by the recipient. Good info 
I would delete it if there are other messages seen, but if it’s the only one, I’d leave it as proof (or take a screenshot).
And contact the postcrossing team, so they can let this person know, it’s not ok to publish messages.