Both Horizon 4 and 3 have a washed out look when hdr is activated on xbox one, x I have halo the master chief collection and Forza motorsport 7, both of which do not suffer from this issue.
I have the same issue on 2 tvs. 2016 vizio p65. 2018 samsung q6nf which i just got. I know some claim this isnt wide spread. I am an av geek and can not fix this. I know sometimes tho i read other people with nicer sets have problems with games that work great for me. Think hdr is new and plays different with different tvs. It looks great in sd too!!