The problem was that the scopes only update if the "Scopes" tab is active.
However, in Blender 2.8 there are no tabs in the image editor anymore.
We might want to check if the corresponding panels are open, but that feels very hacky.
A better approach could be to update the scopes directly before drawing, then it does not matter where they are drawn.
Anyway, let's fix the bug at hand first.