Symptoms
You rebuild a non-clustered index by using CREATE INDEX together with the DROP_EXISTING=ON and ONLINE=ON clauses to add a new column to that non-clustered index in SQL Server. If you add a new column to the nonclustered index, you may encounter blocking in a different SPID when write operations such as INSERT or DELETE are performed on the same underlying table during the index rebuild period.
Resolution
This issue is fixed in the following cumulative updates for SQL Server:
Cumulative Update 2 for SQL Server 2016 SP1
Cumulative Update 4 for SQL Server 2016
Cumulative Update 10 for SQL Server 2014 SP1
Cumulative update 3 for SQL Server 2014 SP2
Cumulative Update 6 for SQL Server 2012 Service Pack 3
About the Cumulative Updates for SQL Server
Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server:
Latest cumulative update for SQL Server 2016
Latest cumulative update for SQL Server 2014 SP1/SP2
Latest cumulative update for SQL Server 2012 SP3
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
References
Learn about the terminology that Microsoft uses to describe software updates.