This is a recent problem in the last handful of driver versions. The problem goes away when you disable Freesync in Radeon Settings.
Driver version 19.1.1RX 480LG 29" Ultrawide 29UM67
Windows 10 1809 x64Displayport
Players I have tested include MPC-HC, MPC-BE, WMP, Films & TV player in W10. The machine is dual booted with Windows 8.1 where the problem also occurs using the Windows 7 driver.