It's such a corner case, i doubt they'd want to promise specific behavior but let's see what Roku* has to say.
Here is another, "legit" (without getting in grey areas of the spec) way to make the lemonade you are looking for: add a single last, N+1 frame from the very end of the video. Actually you can make it a pre-set image that says "no further preview is available" with timestamp=end time. In this way if the "feature" behavior here changes in the future, your BIF will still work since it is complete. Note spec does not require frames to come at fixed time intervals (albeit majority of cases use fixed intervals of say 10sec), they can march to the beat of own drum - so there can be large gap between last two frames.
