Could it be that you are rendering in the background with progressive refinement?

I have a similar problem yesterday. And got the information from the support that rendering in the background with progressive renders is currently not updating the screen until the render finishes. If you don't use progressive refinement it will update normally in background renders. This is an issue the devs are working on fixing and will likely be resolved in the next service release.

If you hit only the render button progressive refinement works and you can stop your render any time.