It does seem that the IPO/liability point is muddy. For example, there was a large pension liability for the whole of Royal Mail that was specifically retained by the government and Royal Mail, as floated, was off the hook there. So there could be the argument that the government arranged things so that Royal Mail became free of the big liabilities. However, the transparency around pensions is a very different animal to the hidden liability of the wrongful prosecutions.
But as you say, the government generally would wish to avoid the Royal Mail IPO be questioned. There isn't one scandal there's a bunch of interlocking ones, and the government and others will engage with the unavoidable scandals, eg the treatment of the subpostmasters (but doing it on the cheap), but would prefer that other scandals get brushed under the carpet. For example, the IPO, ShEx having become captured by Post Office in some ways, and doing its bidding even where it was against the public interest, and the behaviour of the super lawyers, manipulating Post Office, and spending tax-payer money, to play some game of their own in trying to damage Justice Fraser.