I agree with Dave, and i'm pretty sure you have not succeeded on other times.
However, It does not HAVE to be ProRes.
Any I frame only codec will be fine.
For QTchange, it cannot conform to another FPS, no matter the codec. What QTchange can do is set the playback FPS, not the 'original' FPS. (This is in the manual btw...)
(test it with an insane value)
hth,
Bouke
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