I'm having the same issue with my Samsung Q70T. Subtitles are completely useless on The Sopranos. As you said, every other sentence doesn't show.
When I tried watching on my Macbook, more of the subtitles were showing. Seems to be an issue with the TV app, but it's not practical to tether my Macbook to my TV to get around that. Disappointing that this issue has been open for nearly 9 months and it doesn't seem Now have done anything about it. I'm hard of hearing so it's making it difficult to use the service.
Never mind, it's even worse. Watched an episode on my Macbook and that was missing some dialogue and sentences cut off too. Tried to report this on live chat but the agent kept copy and pasting the same spiel about how much Sky care about accessibility (obviously they don't since this report has already been ignored for nearly 9 months). The agent then copy and pasted the synopsis of The Sopranos when I asked for it to be reported to a technical team. Ridiculous.
Over a month in, and despite Now TV's social media team acknowledging the issue, myself providing several examples, and me chasing, this is still not fixed. A manager claimed it was "paraphrasing". I know what paraphrased subtitles are, and in the episodes where subtitles work this is obvious, but in the majority of episodes where subtitles are broken it is clear it's not paraphrasing but broken subs.
The continued behaviour of live chat regarding this issue has also been appalling.