Hi,
As said by David "We have an opening issue on this and will get it fixed for the upcoming release."...
didn't hear anything about the release date so far.
I worked around by
A) Creating an empty scene, add the empty scene to the experience (that works), update / rebuild / change the scene - it seems like only the initial "adding" of scenes with content is the problem
B) .... place the scene into an WebAppBuilder Application, embed the WebAppBuilder Application in the Experience... questionable of course, but works for me until it will be fixed