As far as I know this is just the way the CC was encoded in the file at Britbox. There's likely nothing you can do at your end to fix it. Britbox would be the only ones that can do anything about it.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X
