The mistake is that if you walk backwards it looks like you walk forward. In this report I will make a Video of 1.8 and another of 1.12 and see the change. The truth is I do not think it's a bug, because if you test in real life when you walk backwards you see yourself walking backwards instead, if you walk as in 1.8 then ... you fall in seconds

1.8 : Flat World

1.12 : In the server