Have you tried setting field format in layout mode?
The most common explanation is that text format were applied to the text entered in the field. This can be due to pasted styled text but it can also be done by the user when they are typing in the data. You could select text in the field and select Bold or a different font, for example, and that format will only show in that record.
The solution to this is a function called TextFormatRemove.
You can use replace field contents on this field and use a calculation with this function to strip out all such formatting.
You can use this in a calculated auto-enter calculation to strip out the formatting each time a user attempts to enter formatted data.
You can define a calculation field that uses this function to display the contents of the field but without the formatting--lat allows you to keep the formatting if you need it in other layouts.
With all of the above options, once you've removed the formatting, you can select a text format for the field while in layout mode to specify a format that will be consistently applied to all the records.