This is a known issue documented in this recent video Exploring AMD's Radeon RX Vega 64 Crossfire - YouTube
However, it seems to be known to the general public but it is unknown if AMD is aware of this bug since they have not mentioned it in the release notes.
It is a freesync bug that I talked about in a previous post RX Vega 56/64 Video playback stutters on High Refresh 144hz on 2nd monitor while gaming and also observed by others in this post Vega 56 - Windows 10 Desktop lagging while gaming in windowed mode.
This should be filed as a Freesync bug with multiple monitors.
When freesync is enabled on the main or all 3 displays with refresh rate at 120Hz or higher. Video playback on all web browsers and media players will stutter. Something is wrong with the frame display timing.
Issue exists in 18.7.1 and later
Workaround: Disable Freesync on all monitors and reset PC.
amdmatt can you look into this issue please as it is gaining more and more visibility now that the youtube video has been released. The issue basically renders freesync useless. Exploring AMD's Radeon RX Vega 64 Crossfire - YouTube link to the 11:55 mark where the freesync bug is demonstrated