Ah, I see, got it. Thanks, fixed for next build. It would have waited for you to "finish" the current frame (i.e. switch to another frame or close and reopen) before the preview updated - now the preview is always up to date.
In the current version, switching through different frames doesn't help.
Also importing frames, closing the image editor, reopening it and trying to get a preview still doesn't display the preview.
I just wanted to point that out, in case.