Zombie comment cannot be deleted

A person outside our company made comments in a PDF. They appear as anonymous highlighted text in the Comment pane (using Adobe Acrobat Pro 2017). When I delete one of those comments, it visually disappears, but Acrobat does not register it as a change (that is, the floppy disk "Save" icon remains grayed out). When I close and reopen the PDF, the comment reappears--like a zombie that just won't stay dead. The comments are not locked. I can change some *properties* of the comment, like setting its status to "Rejected" (and *those* changes will be saved), but not other properties like color or author name. I have not found a solution online so far. I cannot post the PDF because it is a confidential client file.