So you actually need a merge field? (If it's to keep the field from being editable, there are alternative methods you can use).
Are you specifying a format for the merge field?
Could the data in the merge field be formatted for some records?
(Data in a merge field can contain text formats that superscede layout specified formatting for the field object. This can be due to pasting formatted text into the field or by selecting the text in an editable field and useing the formatting toolbar to format it.)