I was dead wrong on what the issue was, but the actual explanation does actually hint that that doing error-prone calculation (such as deserialization) to magic

I was dead wrong on what the issue was, but the actual explanation does actually hint that that doing error-prone calculation (such as deserialization) to magically calculate a property is not a good idea. I haven't seen the .json property used before but that only shows that I haven't looked into that many frameworks. I'm not sure why java uses .length for arrays and .size() for collections ... but could it be that it was for similar reasons?