Thanks for reaching out here in the Community!
Does the subtitle issue only occur on Apple TV+ or across other channels installed on your Roku device?
If you haven't tried yet, you can remove the affected channel and then re-install it. To ensure the process is successful, restart your device before adding the channel back in. We would recommend doing this from the Settings menu by navigating to Settings > System > Power (If you do not see a Power submenu, skip to the next step.) > System restart.
For detailed instructions, refer to this Support link: How do I resolve channel playback issues?
Let us know if there's any difference after.
All the best,
Roku Forum Moderator