TLDR: it looks like the disagreement/"renegation" hinges on what level China is going to change its approach to intellectual property. It seems China doesn't want to promise any law changes in the draft agement, insisting on lesser governmental instruments, like decrees.
And if may comment, yeah, this looks like a somewhat silly dispute (if indeed that's all there is to it), but recall that Trump publicly spanked Lighthizer for wanting a MoU instead of a Trade Agreement with China. So, with that in mind, form seems to matter quite a bit in these negotiations.
FWIW, here's a summary of the opposing positions on May 11:
US negotiators earlier accused Beijing of reneging on its commitments, reportedly refusing to change its laws to address US complaints of forced technology transfer, intellectual property theft and anti-competition policies.
But Mr Liu said China did not go back on its word; it only disagreed with the language in the draft agreement.
He also said the US had changed its mind on a sum agreed for Chinese purchases by US President Donald Trump and his counterpart Xi Jinping for Chinese purchases when the leaders met in Argentina last December.
Somewhat similarly, on same day:
Sources told Reuters this week that China had deleted its commitments in the draft agreement that said it would change laws to resolve core complaints of the United States: theft of U.S. intellectual property and trade secrets; forced technology transfers; competition policy; access to financial services; and currency manipulation.
And probably the most detailed (and recent) take on that, on May 13:
Until last week, there were expectations Mr Trump and Mr Xi would sign a trade deal at the summit. However, the trade talks suffered a major setback last week when China proposed extensive revisions to a draft agreement. Beijing wanted to delete prior commitments that Chinese laws would be changed to enact new policies on issues from intellectual property protection to forced technology transfers.
Vice premier Liu He, China’s top economic adviser, sought to defend the changes in talks with senior US officials in Washington on Thursday and Friday, arguing that China could accomplish the policy changes through decrees issued by its state council, or cabinet, sources familiar with the talks said.
US trade representative Robert Lighthizer rejected that, telling Mr Liu that the United States was insisting on restoration of the previous text.
“We would like to see these corrections in an agreement which is codified by law in China, not just a State Council announcement. We need to see something much clearer. And until we do we have to keep our tariffs on,” Mr Kudlow said.
I think that insofar there are no more publicly known details beyond this level as to what the disagreement/"renegation" is about.