To find the issues with the listener connections you may not get complete info in listener log, you need to trace the the listener to monitor the connections, below steps will show you how to enable and disable trace.
[oracle@localhost trace]$ lsnrctl
LSNRCTL for Linux: Version 12.2.0.1.0 - Production on 14-DEC-2017 08:38:21
Copyright (c) 1991, 2016, Oracle. All rights reserved.
Welcome to LSNRCTL, type "help" for information.
LSNRCTL> set current_listener LISTENER
Current Listener is LISTENER
LSNRCTL> set trc_level ADMIN
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
LISTENER parameter "trc_level" set to admin
The command completed successfully
LSNRCTL> show trc_directory
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
LISTENER parameter "trc_directory" set to /u01/app/oracle/diag/tnslsnr/localhost/listener/trace
The command completed successfully
LSNRCTL> show trc_file
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
LISTENER parameter "trc_file" set to ora_9516_140216592200064.trc
The command completed successfully
LSNRCTL> set trc_level off
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
LISTENER parameter "trc_level" set to off
The command completed successfully
LSNRCTL> exit
[oracle@localhost trace]$ ls -lrth
total 212K
-rw-r-----. 1 oracle oinstall 27K Dec 14 08:41 ora_9516_140216592200064.trm
-rw-r-----. 1 oracle oinstall 93K Dec 14 08:41 ora_9516_140216592200064.trc
-rw-r-----. 1 oracle oinstall 19K Dec 14 08:41 listener.log