The reason why trim-to-fill may not work on some frame rates when promoting a clip is due to the difference in the number of frames per second between the source footage and the timeline. When you promote a clip, it’s being interpreted at the frame rate of the timeline, and if the frame rates don’t match, you may experience issues with trim-to-fill.
To resolve this issue, you can try changing the frame rate of the timeline to match the frame rate of the source footage before promoting the clip. You can also try using time remapping or speed adjustments to adjust the timing of the source footage so that it matches the frame rate of the timeline.
Another option is to use a third-party plugin or software that can handle frame rate conversions more seamlessly, such as Adobe Media Encoder or DaVinci Resolve.
Ultimately, the best approach will depend on your specific workflow and the requirements of your project. I recommend experimenting with different methods to see what works best for you.