When importing a schema exported from one 12cR1 AL32UTF8 database to another database with same AL32UTF8 characterset, this ORA -error was generated :
ORA-39346: data loss in character set conversion for object SCHEMA_EXPORT/TABLE/COMMENT
The error message clearly shows the problem data is from “SCHEMA_EXPORT/TABLE/COMMENT”.
Using MDU tool ( Database Migration Assistant for Unicode ) to scan database Dictionary Objects, we can see the problem data is from SYS.COM$.

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Using MDU tool ( Database Migration Assistant for Unicode ) to scan database Dictionary Objects again, we can see all green including SYS.COM$.
Do another export and import again, everything works fine.