Use the Text & Fonts tab of the Modify DGN Export Setup dialog to map Revit text fonts to specific DGN text fonts.
By default, each Revit font listed in the mapping table has a value of {Automatically map font} in the Text Fonts in DGN column, which means the Revit font is maintained in the exported file.
To map a Revit font to a specific DGN text font, click the value for that font in the Text Fonts in DGN column, and select the desired font from the drop-down list.
To change all the values in the table back to {Automatically map font}, click Reset All.