Clicking on the text tool in the ArcMap Drawing toolbar causes the font to change back to the default of Arial, regardless if a different ArcMap supported font was selected beforehand.
The font is not saved according to standards required by ArcMap. The following are possible causes which can render a font unusable.
Use any fontmaking software, for example FontLab, to find the properties of the font.
If none of the above causes apply, please contact ESRI Technical Support.