If Group Members contains more than 50,000 members when synchronized from on-premise to Azure, it will cause errors to be generated in the logs.
The number of members in a group that you can synchronize from an on-premises Active Directory to Azure Active Directory by using Azure AD Connect is limited to 50,000 members as this is hard-coded by Microsoft.
Enhancement Request 115894 has been submitted for consideration in a future release.
Product Management will review the open enhancement requests on a periodic basis and consider them for inclusion in a future product release. Product enhancements will not be considered or implemented in current or prior product releases. There is no guarantee that a specific enhancement request will be implemented in a future version of our product. It is at our discretion if a certain enhancement will be offered for an additional charge or as a chargeable option.