Unable to query remote schema object using partial database link name
I had exactly the same problem: migrated Oracle 11.2 database to new installation 12.1 via DataPump. Database links in then 12c only worked when accessed with fully qualified names, but the application had been coded using only their partial names.
The problem was the domain of the new database's Global Name was different - see http://www.dba86.com/docs/oracle/12.1/ADMIN/ds_admin.htm#ADMIN029 (search for 'partial' within the dblinks section).
I created additional dblinks with fully qualified names using new domain name,  by extracting the ddl from the 11.2 database: 
SELECT DBMS_METADATA.GET_DDL('DB_LINK',a.db_link,a.owner) FROM dba_db_links a;
 - see excellent hints from
 Alex Lima https://oraclespin.com/2008/03/28/how-to-extract-the-db-links-ddl-with-the-password
and Simon Krenger https://www.krenger.ch/blog/find-password-for-database-link/