MovieTexture on Linux with Unity 5.3.4

I'm getting no joy with MovieTextures on Linux with Unity 5.3.4. Things are fine on windows with a 1920x1080 MovieTexture. It plays just fine (think background movie on the main menu of a game). The movie texture in question is assigned to a RawImage UI texture property. There is no fancy downloading or anything like that to get in the way.

On Linux, it won't play. Inspection of the MovieTexture shows that it reports a 16x16 size (not 1920x1080) and isReadyToPlay reports false forever. Using -force-opengl, or not, doesn't make any difference. Things were fine in Unity 5.2.