Symptoms
Assume that you create a primary key on a column which includes large decimal values or numeric values in Microsoft SQL Server 2012, 2014 or 2016. Then, you create a full-text index by using this column as the unique key index. In this situation, if there are some rows that are failed to be indexed, the full-text key value will be record as a negative number or unicode characters. Therefore, you can't identify the rows which are failed to be indexed.
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 6 for SQL Server 2012 Service Pack 3
Cumulative Update 10 for SQL Server 2014 SP1
Cumulative update 3 for SQL Server 2014 SP2
About 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
Workaround
To work around this issue, add a unique bigint or int column to the table and specify full-text by using that column instead. Int and bigint correctly report their values to the full-text error log when a failed row or document is reported. The unique column used by full-text doesn't have to be the primary key of the table.
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.