bochs will lose it's font if the screen is refreshed in certain ways.
I don't know what causes it, but I can tell you how to trigger it.
Boot bochs from FreeDos CD, click reset button. You can see the blue
rectangle normally displayed by FreeDos, but all text is missing. You
can still press "1" at the boot prompt to boot FreeDos, you'll see the
cursor fly by like mad, but still with no text.
You can also boot from a MS-DOS boot disk. fdisk /status will cause all
text to be lost.
It can probably be triggered any number of other ways.