Hi,
This is the expected behavior, and this has nothing to do with adaptive degradation.
The preview changing to wirebox for a brief moment when changing the frames on timeline is because of asynchronous loading of proxies and textures, which makes loading times much faster.
If you hit play, you will see the animated proxies in wirebox, as there is no enough time between frames to load the set preview type.
Unfortunately this is a drawback coming with faster loading times.
You were probably using Corona 7 or older.
Hope this helps.