That’s because I did a test with a new Amp in the Simulacrum and found out this issue. Instead of Machinery, void damage deals only half damage to robotics. I don’t know in which update did this change.
If it's only an exception to the rule (and not a majority), it's fine. E.g. Ancient Healers don't take reduced damage despite being Fossilized. Please continue your tests though, if you aren't sure yet.