British Airways crew have been issued with instructions on how to use this harness which has been approved by the CAA for use on British Airways flights.

I think you can put this down to human error that the strap was fitted in an incorrect manner on this occasion and not picked up by the operating cabin crew.

As already mentioned by others, although this was not the correct way to fit it, opening the tray table would have just tightened the strap around the seat, rather than affect the harness that goes around the child. So no harm or danger in this particular case of human error.