Final Cut Pro cannot render Speed Ramps with Optical Flow

Title says it all. This is a shocking bug on multiple levels:

1. Any speed ramp is going to use optical flow, because the results are terrible otherwise. (in contrast, a fixed retiming, e.g. 50% or 200%, can potentially get away with simplistic methods like frame blending or skipping)
2. This bug has been around for at least 13 years. There are reports at least as far back as 2013 (if not earlier) about this problem, for example. In Apple’s supports forums. Which just goes to show how utterly useless it is to report problems in Apple’s support forums.
   
   Final Cut Pro X – the infamous rewrite which almost killed the product by alienating half the users, but which also introduced optical flow & speed ramping – was released in 2011. I have a suspicion that this bug was introduced then, right from the start. Those two headlining new features, conspicuously introduced and marketed together in that 2011 release, have likely never worked together.

Since you cannot render, you can neither preview it (since Final Cut Pro cannot render optical flow retiming in real time) nor export video containing it.

I can’t say with certainty that this always happens – maybe there’s some hidden factor – since while it does for me, there are anecdotes online saying the bug mysteriously manifests in some projects but not others.

In any case, I’m just dumbfounded. I realise Final Cut Pro has not had the healthiest staffing or corporate support since pretty much ever, but that such a simple, important bug remains after more than a decade… what on earth are you doing, Cupertino?