Can't help you without a sample.
All it would take is one row from your dataset which has a problem with madeup headers .
The weird thing is, when I do that it works. I copied a bunch of rows that were having issues into a new text file and it came up fine. So then I tried copying the whole file into a new text file and it didn't work.
Another extremely odd issue is when I save the file as an xlsx and I try to change a numerical field to a string it changes all of the numbers to "ge0eral". When I try to isolate this in a sample file it fixes itself as well. There must be something corrupted with that data that I just can't figure out because I have done this exact same procedure countless times and never seen anything like this.
I'm guessing you've got line break characters in your data somewhere. Like a comment field from a web application or something where someone can effectively press enter and it creates a new line. This can be misinterpreted by Tableau or Excel and it can cause it to all shift down a line at that point.
Make sure you're using a text qualifier in Tableau when you connect to the file.