Thank you for your post.
I do not know why this is happening. The only way I can determine why there is a problem is to see a clone of the file. I have sent you a private message (top of this page - right side - X Messages) with instructions where to send the clone.
If you are unable to send a clone (due to confidentiality), then I would assume this layout may be corrupt. Therefore, I would try creating a new layout with this text field. First, I would try it as a plain text field, and then I would try it as a drop down list.