Problem with displaying a value digitally

When you say the font become unreadable, does it appear that your letters are just becoming mangled garbage? I believe what is happening is, if you are changing the font of your text mesh, but you are not updating the material/texture your text mesh is using, your font will become unreadable, as it is using a font type and a material that do not match.

Based on the scenario you described, I'm assuming you're changing your font through script at runtime. Try also updating the material of your text mesh with the material of the font when you assign your text mesh a new font.

You can also test this in the editor while the game is not playing as well to see if this is your issue. If you have a text mesh object you can access, drag a new font onto that object in the editor and watch how the font changes. If you drag that fonts material onto your object as well, you should see it update and become readable again with your new font type/size.

Hope that helps