First of all, it’s kind of infuriating that Apple used a term for their variable refresh rate technology that’s effectively impossible to search for.
I know ProMotion 24hz support was added to Infuse a few years ago; did Apple break that functionality at some point? I tried a 24hz video, which the Playback HUD has shown as “Source frame rate: 24.00 fps”, but playback never changes from “Screen frame rate: 120hz/120.00 Hz/120.00hz”
Is there any way to force/enable a 24hz mode for this kind of content?