To test, either have ksh as login shell, or create a test user.
Then ssh localhost.

Sample session:

---8<---
$ cd /tmp/alan
$ cat tst.ksh
#!/bin/ksh

D=$(date)
echo $D > /tmp/alan/tst.dat

$ ls
tst.ksh
$ (ksh /tmp/alan/tst.ksh &)
[1]	28190
$ ls
tst.ksh
---8<---

This happens due to a double fork, once for "( cmd )" and
then the first fork forks again due to the "&", but, just
when the child branches, it receives a SIGHUP and
immediately exits.

I tested a quick&dirty patch, that basically reverts to
rhel5 ksh behavior, and there are no regressions in the
test cases.

---8<---
diff -up ksh-20120801/src/cmd/ksh93/sh/xec.c.orig ksh-20120801/src/cmd/ksh93/sh/xec.c
--- ksh-20120801/src/cmd/ksh93/sh/xec.c.orig	2015-04-28 16:55:25.521818636 -0300
+++ ksh-20120801/src/cmd/ksh93/sh/xec.c	2015-04-28 16:56:34.037753565 -0300
@@ -2059,16 +2059,20 @@ int sh_exec(register const Shnode_t *t,
 			}
 			else if(((type=t->par.partre->tre.tretyp)&FAMP) && ((type&COMMSK)==TFORK))
 			{
+#if 0
 				pid_t	pid;
 				sfsync(NIL(Sfio_t*));
 				while((pid=fork())< 0)
 					_sh_fork(shp,pid,0,0);
 				if(pid==0)
 				{
+#endif
 					sh_exec(t->par.partre,flags);
+#if 0
 					shp->st.trapcom[0]=0;
 					sh_done(shp,0);
 				}
+#endif
 			}
 			else
 				sh_subshell(shp,t->par.partre,flags,0);
---8<---

I also asked upstream, see http://lists.research.att.com/pipermail/ast-users/2015q2/004754.html
but actually, there is a bad comment there, about crashes if
running it as "( cmd ) &". Likely it was due to the way I was
debugging it.

Note that if one has bash as login shell, then starts, or exec
ksh, or even if has ksh as login shell, and first exec ksh,
it will work, because then SIGHUP will be blocked in the
"orphan" process (orphan due to double fork).