You might have noticed a warning at the top of the release notes for SQL Server 2016 SP2 CU 16:
Note: After you apply CU 16 for SQL Server 2016 SP2, you might encounter an issue in which DML (insert/update/delete) queries that use parallel plans cannot complete any execution and encounter HP_SPOOL_BARRIER waits. You can use the trace flag 13116 or MAXDOP=1 hint to work around this issue. This issue is related to the introduction of fix for 13685819 and it will be fixed in the next Cumulative Update.
That warning links to bug reference 13685819 on the same page. There isn’t a separate KB article, only the description:
Fixes an issue with insert query in SQL Server 2016 that reads the data from the same table and uses a parallel execution plan may produce duplicate rows