ksh: file to string expansion fails if ksh called with stdin closed

Issue

• String expansion fails if ksh called with stdin closed.
• Refer scripts below to reproduce issue i.e. "demo" and "demo.sub"

"demo" script contents:

    #!/bin/ksh
    exec <&- 2>/dev/null
    $0.sub


"demo.sub" contents:

    #!/bin/ksh

    tmpfile=/tmp/wrk-$$
    echo  "Hello World" > $tmpfile
    wc -c $tmpfile

    a=$(<$tmpfile)
    echo ${#a}

    a=$(<$tmpfile)
    echo ${#a}

    rm $tmpfile

• Script does not work

[root@vm201 mtest]# 
[root@vm201 mtest]# ./demo
12 /tmp/wrk-1025
0
11

• Without closing stdin in main (demo) script all works as expected.

[root@vm201 mtest]# more demo
#!/bin/ksh
#exec <&- 2>/dev/null
exec 2>/dev/null 
$0.sub

[root@vm201 mtest]# ./demo
12 /tmp/wrk-1020
11
11