I can reproduce this in Windows 7.
As a work around (not suggesting that you don't have a bug here), you can set up an auto-enter calculation that strips out the tab characters.
Substitute ( self ; Char ( 9 ) ; "" )
Thank you for the post.
I am able to replicate this behavior with FileMaker Pro 12.0v4 (Advanced). Both Windows 7 and Mac OS X 10.8.4 appear to be affected. Since this was not the same behavior in FileMaker Pro 11, I've forwarded a report to Testing and Development for review.