Update on the situation: Windows media buttons in Plex for Windows are functional while in-game in Warframe, and I don’t know why. Maybe Windows is treating the application differently in games?
Other fullscreen applications that I use (e.g., Netflix Windows Store app) seem to not be able to do this, not sure why Warframe in particular is “fixing” my issue.
Edit: According to this post, Plex for Windows doesn’t support media buttons. So that sucks. The only option is to try Plex Media Player, for which the development has apparently been discontinued except for bug fixes.
