I noticed this in Beta 3 as well, but do you have more than 1 type of file name tabs?
First of all, I have the "File Tabs" showing, and docked at the top of the edit window.
Now if I take the Edit window and split it (horizontal or vertical doesn't matter) I suddenly have 2 tabs.
The square ones from the File Tabs, and slanted sides tabs on the split windows.
Close the split window again, and now I have 2 sets for all open files.
How do I hide/show the "slanted sides" version of File Tabs?
(I actually like those but don't know how I can get them to turn on)