I report too the same behavior for RX 460 in windows 10 versions 1903, 1909. Any driver published after 1903's release date it seems to have this problem of preventing monitor to go to sleep mode. The most recent version of amd radeon driver that worked flowlessly was 18.2.3, but it can't be no longer kept in the system because windows 10 will update it automatically to a newer version released in 2019. I disabled the AEEU service and solved. But that's not professional, AMD!