[Help-bash] Command Substition (using tail) in exec line stopped working

[Erik Ackermann]

So I just posted a more detailed, formatted version on stackexchange, but
wanted to reach out here as well, particularly since this might be a
regression, or at least a new behavior I was not aware of.

http://unix.stackexchange.com/questions/316479/bash-command-
substitution-in-exec-line-some-commands-stopped-working

Basically though, this command works (script.sh writes to /tmp/log):

  exec ./script.sh >(cat > log)

While this one does not work (file created but no contents):

  exec ./script.sh >(tail > log)

Where script.sh contains:

  #!/bin/bash
  echo "fd: $1" >/tmp/out
  ls -l "$1" >>/tmp/out
  echo "SUCCESS" > "$1"

In both cases /tmp/out shows a writable fd.

It's a boiled down example of something that's been in my .bashrc for a
long time, but just noticed it stopped working on my machine recently. Is
there something special about the tail/head commands that makes this not
work..?

Bash version: GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)