Bitmap fonts are very predictable. If the character exists in the texture, then it will be used. If it doesn't exist, then it won't be.
If the text becomes garbled for any reason, then the font's definition and the texture are no longer in sync. It can happen if you update the atlas where the font resides, changing its texture's size outside of using the Atlas Maker, or if you create the font using different exported FNT + Image files.