Perhaps a side note - but I know a case where someone whose code had a lot of bugs and design problems but when they were with the company they worked very hard

Perhaps a side note - but I know a case where someone whose code had a lot of bugs and design problems but when they were with the company they worked very hard and would address problems - often with band-aid solutions but at least they took care of their own messes so it didn't fall onto others. All of this hard work and long hours and diligence made them seem to management as if they were amazing. However after they have left, they are obviously no longer supporting their terrible code and the ones who had to take it over will certainly demonize the departed - it's a bit unfair but someone who isn't helping fix their own mess (even if they have a good reason - they moved on) is still a huge negative. You are right though, this is somewhat a biased perception and scapegoating is real.