I don’t use the HEX much, and when I do I’ll almost always buy through necessity as I walk to the station, so consequently I’m probably more prone to make an error when booking using the app.

I managed to do just that very soon after the change took place - I only noticed when I was on the train and it was about to leave, so no time to remedy anything.

The ticket inspector noticed, mentioned it, but couldn’t care less and just validated the ticket. Whether that’s the case a year on is definitely a case of YMMV, but it probably does show that as an operator there is a certain amount of latitude to make good for those who clearly have been careless rather than deliberately circumventive.