Hi @mtennant thanks for your post here in the Community, hoping you're well.
As @newapollo has mentioned here, it's not possible to have Subtitles synchronising with Live TV, purely for the fact they are being generated at the exact time as the event goes on, the Subtitles themselves are live in nature.
It really wouldn't be possible for any TV Box (including elsewhere) to do this for Live TV, with exception if the event was perhaps a repeat (but played as live) if that makes sense.
Even though this doesn't help, I hope it brings you some closure to this issue.
Many thanks
Tom_W