How to reproduce?
- Go to a wiki with Extension:Thanks installed. Open the revision history of a page. Press the "thanks" link and confirm.
- The fact that a thank you message has been sent will be publicly logged including sender and receiver.
- The "thank you" will be displayed to a user but is not publicly logged.
A "thank you" is, despite it's just a button, always a personal message. It's totally unexpected to be publicly logged, if not even a privacy violation. There is no indication when sending "thanks" that this action will be logged publicly.
T90483 is a site request for turning the thanks log off.
For context on why the feature works like this, see T51087: Specify which edit was thanked in Special:Log/thanks, both for private and public records' sake, if configured to do so (we do not specify the edit, but we do specify the sender and recipient to prevent abuse, as explained there). There are also several bugs (duplicates of that) requesting we add more information to the public log.
A previous request to disable this logging was declined at T57428: Remove all logging for the Thanks extension.