With the final irony probably being that the acquiring company may lose its taste for the acquisition given the dispute, meaning they are just fighting over who gets half of the ashes of a bonfire of money.
I suspect large corporations are no strangers to these lawsuits and if they really wanted the IP, they wouldn't be scared off by debacle...although they would likely double down on any due diligence they planned.
This is not uncommon. The deal will have an escrow where money is held for a time period to protect against such claims.