We're experiencing exactly the same issue with animated proxies.
Almost impossible to export previews from Viewport because of this bug.
Funny thing is that we did an animation last year with the same proxies and it was working smoothly.
Now, updated to Corona 8, we have this problem.
Any known solution?
Unfortunatelly I got this answer from support team:
"The "flicker" between modes is because of the asynchronous loading introduced in Corona 7. This makes the textures and proxies be loaded much faster but may come with such drawback.
I will see what is possible to report about this and make it better.
But for now using Corona 7 and newer, you may probably use the workarounds I proposed."
Workaround is generate viewport before convert into proxies, but for me it´s impossible to do it.