It sounds like the TEXT function in Excel is not recognizing the month number and is defaulting to "Jan" for all values. This could be due to a formatting issue or a language setting in Excel.

Here are a few steps you can try to resolve the issue:

1. Check the formatting of the cell containing the month number. Make sure it is formatted as a number and not as text. You can do this by selecting the cell and going to the Home tab Number group Number format dropdown.
2. Check the language setting in Excel. Go to File Options Language. Make sure the language is set to the correct language for your region.
3. Try using the MONTH function instead of the TEXT function. The MONTH function returns the month number as a value between 1 and 12, which you can then use in a VLOOKUP formula to return the month name. For example, if the month number is in cell A1, you could use the formula =VLOOKUP(A1,{1,"Jan";2,"Feb";3,"Mar";4,"Apr";5,"Ma y";6,"Jun";7,"Jul";8,"Aug";9,"Sep";10,"Oct";11,"No v";12,"Dec"},2,FALSE) to return the month name.