Many jurisdictions do, and all jurisdictions should, test every machine with a reference set of ballots and verify the correct output as part of commissioning. This is intended to detect errors of this type.
And it's probably easy in the firmware to check if you're being tested (ask Volkswagen). And even if it's not, you could write a firmware you could trigger with a specific ballot.
The bottom line is firmware for these things should and must be open so they can be audited by experts. And hackers must be given access to them so they can try to do their worst.
As long as you treat it as a black box, I'm afraid to say you aren't trusting the device, you are just having faith in it.
Generally, from a network security perspective, electronic pollbook systems are a much greater concern as they are connected over the internet as a matter of standard practice, and it's difficult to avoid this while preserving functionality. There are also more, smaller vendors for EPBs and they receive significantly less scrutiny or security review than tabulators.