Background process in ksh command substitution hangs

Issue

Since upgrading ksh93 to ksh-20120801-10.el6_5.3 it is no longer possible to send a process to the background out of a command substitution. Here is how it should work:

# time ksh93 -c 'out=$(/bin/sleep 2 >/dev/null & echo ok); echo $out'
ok

real    0m0.011s
user    0m0.000s
sys     0m0.005s

sleep is send to background and the whole command immediately returns. This works because the redirection to /dev/null releases sleep from the pipe of the command substitution $(...). And here is how it behaves with the latest patches (problem still present in ksh-20120801-10.el6_5.4):

# time ksh93 -c 'out=$(/bin/sleep 2 >/dev/null & echo ok); echo $out'
ok

real    0m2.010s
user    0m0.001s
sys 0m0.005s

sleep blocks the entire command for the two seconds, which is wrong and breaks scripts.