For some reason, it is no longer possible to edit the names and the font of the tabs of Tab Control 1 directly through the FP. I didn't notice when this behaviour appeared but I may have changed some "advanced" properties. Unfortunately, all my trials to make the tab names editable again remained unsuccessful.
As you can see in the example, everything works fine when using a new tab control. An easy solution would be to replace the tab control but I would really like to know why Tab Control 1 behaves like this.