Hi,
I had the same issue with Visual Stusio 2008. For me it seems there is a bug in the form designer.
The solution was, to delete and recreate the TextBox control.
I also double checked the properties of the "corrupt" and "good" control. Both had the same. But some proteries were displayed bold for the corrupt control. I did not investigate deeper because it worked fine for me, if you like to do so, I would propose to go the same way.
Create new TextBox control
Set all properties like the corrupt one
compare the properties view, and check/compare the code created automatically
by the designer.
Good luck and Regards,
Frank