Why is alert.log not located in path indicated by BACKGROUND_DUMP_DEST in oracle 12c (12.1.0)
BACKGROUD_DUMP_DEST parameter specifies the path where debugging trace files for the background processes like PMON, DBWn etc. 


  BACKGROUD_DUMP_DEST
  
  This parameter is ignored by the diagnosability infrastructure introduced in Oracle Database 11g Release 1 (11.1), which places trace and core files in a location controlled by the DIAGNOSTIC_DEST initialization parameter.


The alert logs, trace files are  found  in Automatic Diagnostic Repository (ADR) Home determined by DIAGNOSTIC_DEST.
&lt;diagnostic_dest&gt;/diag/rdbms/&lt;dbname&gt;/&lt;instname&gt;.

The alert log location is &lt;diagnostic_dest&gt;/diag/rdbms/&lt;dbname&gt;/&lt;instname&gt;/trace/.


  DIAGNOSTIC_DEST
  
  Derived from the value of ORACLE_BASE ($ORACLE_BASE). If the $ORACLE_BASE environment variable does not exist, then $ORACLE_HOME is used.


The following files are located under the ADR home directory:
Alert Logs
Trace Files
Core Files
Incident Files

Reference:DIAGNOSTIC _DEST