It's a bug. The tab is not supposed to show its caption when hovering its content. It will be fixed in 5.10.
I already reported this in Ticket 22346 which was about incorrect or missing hover text. In the last couple of releases, functionality has been added to buttons etc. to define a property 'Tool tip' text. This works fine to display the correct hover text or no hover text, but there is currently no such functionality for tab containers.
As feedback to this ticket I added a suggestion that this property should also be added to tabs so that you can define the hover text, or have no hover text. The ticket was Closed by Mendix, so I have no idea whether this feedback has been seen or acknowledged.
It might be worth creating a new request for this added functionality.