Thank you for sharing this behavior. That does seem like it is behaving oddly. Can you share the URL to the Tableau Public viz that has this occurring. I can share the URL with the Tableau Public to investigate the cause.
Doing some research I did not find other reports of this occurring for other users.
Look forward to hearing from you.
I've raised the matter with Phil N & Sean Boon. The issue is that Dashboard (and tooltip) transparency is not yet enabled.