Mueller’s team would have discussed these and every other subject relating to the Russia investigation with Manafort. The fact that Mueller decided to take the significant step of stating that Manafort “breached” his agreement by telling lies and committing crimes implies that the subjects about which Manafort lied were not minor but were lies about facts central to the Russia investigation. It also implies that Mueller has good evidence of those lies that he could submit to a judge to defend that allegation.
So, did Manafort continue to lie despite that evidence because he remains loyal to Trump? Does he fear confirming that there was conspiracy with Russia because it may implicate powerful people here in the United States and/or powerful people abroad? Or does he fear losing a chance at a pardon if he provides the bridge from the Trump campaign to indicted Russians?
We don’t yet know the answers to these questions, but we might soon find out: if Mueller chooses to file his “detailed sentencing submission” publicly (although the judge could allow the filing under seal for some period of time if there is good reason to continue keeping information non-public) or if he charges other individuals should evidence of Manafort’s lies also implicate people in conspiracies.