Thank you for your post.
I am unable to replicate the issue using FileMaker Pro 13.0v1 and FileMaker Pro 13.0v2 under Mac OS X 10.9.2 and Mac OS X 10.8.5. I have labeled the tab controls exactly like your example.
In case your original Tab Control is damaged, go into Layout Mode, ensure the Inspector is showing, click the Position tab, click the Tab Control, and write down the measurements.
Next, create a new Tab Control somewhere to the right of the original tab panel with the same height and width. Then, enter the same names for each of the tab panels. See if the tab panel information looks correct. If so, then copy the objects from the original tab panels into the new tab panels. Go to Browse Mode to verify the objects are correct. Finally, remove the original tab control, and set the position of the new tab control to the position of the original tab panel.
Keep me updated with any progress.