The answer to many physical security questions is "it depends." I don't have my materials on me anymore, but in general, seal tampering means a lot of extra scrutiny on the people watching the machines and transporting them. The chain of custody will pin the blame on the last person who signed off, and things get investigated as needed.
The system doesn't have something in place typically that says "if (sealVoided) { throw out election }" it just means that additional precautions are taken to ensure everything is good. It's never a binary answer, unfortunately.
Internally applied seals would be one form of defense against this vector.
Breaking the external seals would put the device into the "needs further investigation" category. After the election the device would be inspected and the internal seals confirmed. If those were still intact, the results from the machine could be certified.
The system doesn't have something in place typically that says "if (sealVoided) { throw out election }" it just means that additional precautions are taken to ensure everything is good. It's never a binary answer, unfortunately.