In standalone/domain start scripts (standalone.sh/bat) the value of environment variable JBOSS_LOG_DIR is checked for:

# determine the default log dir, if not set
if [ "x$JBOSS_LOG_DIR" = "x" ]; then
   JBOSS_LOG_DIR="$JBOSS_BASE_DIR/log"
fi

However, this is not actually used to set the value of Java property jboss.server.log.dir.

-Djboss.home.dir="$JBOSS_HOME" 
-Djboss.server.base.dir="$JBOSS_BASE_DIR" 
"$SERVER_OPTS"

(It should be set at the same place)

This leads Jboss/Wildfly/EAP to assume the default value of $JBOSS_BASE_DIR/log.

This is a problem for those who want to override the location of the server.log files.