Customers currently in the process of finalizing a migration from legacy Public Folders in Exchange 2010/2007 to Modern Public Folders in Exchange Online or Exchange 2016/2013, should read KB3161916. We’ve identified a scenario which, although it doesn’t appear to be common, could result in a loss of data. If you have previously completed a migration of legacy to modern public folders and believe you may have encountered data loss, please contact support services for additional assistance.