I've confirmed the following with 0.43:
At first I thought 0.43 had the same issue with interpolation not starting. GRisk, you were right that when I uncheck "Don't interpolate scene changes", everything starts normally.
Then, I realized that "Don't interpolate scene changes" actually does work if I wait long enough. With that setting enabled, it took hours after PNG frame extraction completed before interpolation started. With "Don't interpolate scene changes" disabled, interpolation starts immediately.