I don't think this is a good example of robustness: if the change had happened to affect the DER encoding instead of just a field within the DER encoding, this

I don't think this is a good example of robustness: if the change had happened to affect the DER encoding instead of just a field within the DER encoding, this could just have easily been an example of the fragility of binary formats. Rather, what's happening here is that the interpretation of the parsed format is malleable in an arguably incorrect way. There aren't any great solutions that I'm aware of to that, since the "fix" is to validate the correctness of fields that ultimately don't matter to the operation at hand.