I realize this is an old topic but I’ve recently had one of those…. hair pulling moments of frustration.
Anyhow, my workflow always requires multiple levels of revisions, so prerenders don’t quite cut it for me. What I’ve found to solve this issue is rather simple – setting all keyframes to HOLD.
Of course this could open a whole new can of worms for you depending on what exactly you’re stretching, but it’s always a handy trick for the back pocket. Thinking about what the software is doing – basically remapping the time between keyframes – it’s going to assume you want the tweens to operate in kind. Hold keyframes preempt this.