ksh shell script doesn't wait for the background processes if executed remotely via ssh

Issue

• Consider following script on remote server:

$ cat test.sh
#!/bin/ksh
echo "`date` : EXECUTING $0 on `hostname`"

grep -ri "TEST" /* >> ~/test.out &
echo "`date` : FINISHED $0 on `hostname`"

and local script:

$ cat testwait.sh 
#!/bin/ksh

for i in localhost
do
    ssh $i  ~/test.sh &
done

wait
echo "FINISHED"

which executes test.sh remotely via ssh.

• If you are running the script in RHEL 5.4 and higher, it shows following output:

$ ksh testwait.sh 
user@localhost's password: 
Tue Feb 11 16:25:58 EST 2014 : EXECUTING /home/user/test.sh on localhost.localdomain
Tue Feb 11 16:25:58 EST 2014 : FINISHED /home/user/test.sh on localhost.localdomain
FINISHED

and exits immediately.

• On RHEL 5.3, the progress of grep is shown:

$ ksh testwait.sh 
user@localhost's password: 
Tue Feb 11 17:01:09 EST 2014 : EXECUTING /home/user/test.sh on localhost.localdomain
Tue Feb 11 17:01:09 EST 2014 : FINISHED /home/user/test.sh on localhost.localdomain
grep: /dev/vsock: Invalid argument
grep: /dev/vmci: Invalid argument
grep: /dev/gpmctl: No such device or address
grep: /dev/log: No such device or address
...

...
FINISHED

and script waits until grep finishes.