This is an issue that has been in place for years.

On any chat window with System messages turned on, if you scroll back beyond the latest 2 pages or so the buffer will be filled with any and all System messages regardless of whether the window is configured to filter them out.  If I'm scrolling back to look for a message that I have enabled (like Private Item or Worker) I sometimes find duplicate messages.

It's almost like the chat scrollback buffer is meant to be longer than it is, but when we scroll back we run out of data and there's some unfiltered garbage there.