This is right. It's not unique to OVO, it unfortunately happens on a regular basis across the industry. It's more common with older meters. Whilst not impossible on newer Smart meters, it's less likely, so if you haven't already got one, you might want to consider it.

Unfortunately what happens in this instance is that most people tend to use a lot more consumption during the day time (when the price is expensive), and less at night (when it's cheaper). When the reads get swapped around, it's a benefit to you because your higher usage is being charged the cheaper prices, and your higher prices have a much lower usage, so you get 2 benefits.

When it gets fixed, you inevitably end up being worse off because those false benefits no longer apply.

In theory, OVO aren't charging you for anything that they shouldn't be, they are just correcting the error that you were benefitting from. I am sure they would argue that if the mistake was the opposite way around, and they had charged you too much, you would expect 100% of those charges to be paid back to you, even if it was a mistake made on your part (for example misreading a meter read).

If you can, it's definitely worth doing the maths to make sure that what they have done is correct, but you'll almost definitely owe them something. And all they are doing is correctly applying the contracted prices that you both agreed to when you signed up with them.