There are two levels at which text in a field can be formatted. The text can be formatted at the data level in individual records. This can be done on Mac systems if formatted text is pasted into the field. It can happen in either system when in browse mode if you select the text and use the formatting bar to apply a format such as a specific font and/or font size.
The text can be formatted at the layout level. This is done by selecting the field while in layout mode and then specifying a text format. Conditional formats are also applied at this level.
Formats applied at the data level supersede formats applied at the layout level. To see the layout level formats, the formatting applied to individual records at the data level must be removed.
You can strip data level formatting from existing records by clicking into the field, selecting Replace Field Contents from the Records menu and using this calculation: TextFormatRemove ( YourTable::YourField )
You can prevent this from happening in future cases, by adding this expression as an auto-entered calculation: TextFormatRemove ( Self ) and clear the "do not replace existing value..." option.