A new version of Azure AD Connect was released, the current build is 1.1.882.0.
This update fixes just a small issue when upgrading from an earlier version while having the AADC DB hosted on Always On SQL Server.
NOTE! This release is only available for download and will not installed by auto-update feature.
See the announcement here: https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-version-history
Azure AD Connect Upgrade fails if SQL Always On Availability is configured for the ADSync DB. This hotfix addresses this issue and allows Upgrade to succeed.
You can download it from here https://www.microsoft.com/en-us/download/details.aspx?id=47594