[Quest] Weird Canvas Render behaviour

Weird one - canvas rendering fine in the editor, but when the scene loads in the build the canvas won't render (all canvas is black, rest of scene is fine)

however, on rollover i can still click a button - and when i do, the canvas moves into the configuration setting for that button and the render works fine from then on. 

so i'm guessing there is something in the event system that is "activating" the proper rendering, but i cant see what it is? 

also - i'm using URP!