Unable to control media player (except for volume)

I've just reinstalled HASS.agent, updating to the latest 2.1.0-beta1 on both PC and HA, and am able to see my media_player in HA, including cover art and progress bar. However, I am unable to use next, previous, or play/pause. Tried in Tidal, Windows Media Player, and YouTube Music (progressive web app). Volume controls are the only bit that work. Any ideas on what to look at?