With frame rate matching turned on, any Dolby Vision content causes the Apple TV to switch to 60hz, even if the content is 24 hz content.
To test this, I can use Netflix to open any Dolby Vision content. It will play at 24 hz on any Netflix app except the Apple TV Netflix app, which plays at 60 hz (giving the unnatural "soap opera effect" look).
The same is true for any iTunes movie I have redeemed that has Dolby Vision. It plays at 60 hz on Apple TV with the "soap opera effect".