PRINTMISS, DD is a subtitler so I put your problem to her. Her answer is as follows:
It is definitely something to do with the how the programme has been recorded because it will not have been transmitted like that.
"Obviously I do not know how the person with the problem was recording the programme, – it’s possible it could be a fault with their recording device, or their digital box or something wrong with the signal the BBC or ITV or whatever send out. If it was Mr Selfridge, an ITV show, with BBC subtitles, then I cannot see that the problem would be with the broadcaster because two of them are involved. I’m going to guess that the recorder, if it’s separate from the digital box, would only record the data it’s sent, rather than change it, so it seems most likely to be something to do with the digital box or digital platform itself.
Once you record a programme with subtitles, the two things are kind of sewn together so once the gadget has got confused and recorded the wrong subs with a programme, then they won’t be removable or changeable. However, if it’s a BBC programme, I think most things on the iPlayer have subtitles so if they really want to see the show with subtitles then iPlayer could be a back up plan."
I hope this helps.