Was wondering about this after reading the news about Jimmy Carter casting his vote (since statistically, at 100, death is a possibility). Turns out that the answer depends on the state, and there's not even a consistent pattern.
Some states explicitly say those votes count, and those include Democratic strongholds like Massachusetts and Maryland and Republican ones like Montana and Florida. States that explicitly reject them again range politically from Delaware to Mississippi. And a few (NY, Colorado, Kansas) allow votes to be challenged. Meanwhile, most states (including GA) have literally NOTHING on the books either way, which just sounds messy and a great way to have things get horribly confusing.