The UK–I think Boris Johnson specifically–is accusing China of covering up tens of thousands of deaths.
Here is my guess: they made the same mistake Italy did, of quarantining the young with the old. This actually worsened the situation. It was bad policy that they don’t now want to admit.
One of the main reasons the Spanish flu was so deadly was that it got into confined quarters. If the disease is already inside the gates, a quarantine essentially guarantees everyone in that space gets it. And if old are in the mix, many of them die.