Why is this inconsistent? I would expect here docs and here strings to be functionally equivalent to redirecting input from a file.

$ bash --version
GNU bash, version 4.1.2(1)-release (x86_64-unknown-linux-gnu)
...

Expected output:

Prints each item from list.

$ for server in $(<servers.txt); do echo ${server}; done
server1
server2
server3
$

Unexpected output:

Prints nothing.

$ for server in $(<<EOF
> server1
> server2
> server3
> EOF
> ); do echo ${server}; done
$

Unexpected output:

Prints nothing.

$ for server in $(<<<"server1
> server2
> server3"); do echo ${server}; done
$

Edit: I traced bash while using an input redirector, and again with a here string. Here's the difference in behavior between the two:

Input redirector

$ echo $(<servers.txt)

...
31929 open("servers.txt", O_RDONLY)     = 3
31929 read(3, "server1\nserver2\nserver3\n", 128) = 24
31929 write(1, "server1\nserver2\nserver3\n", 24) = 24
...

This spawns child process 31929, which opens servers.txt as FD 3 and writes it to stdout.

Here string

$ echo $(<<<"server1
> server2
> server3")

...
31990 open("/tmp/sh-thd-106091305575", O_WRONLY|O_CREAT|O_EXCL|O_TRUNC, 0600) = 3
31990 write(3, "server1\nserver2\nserver3", 23) = 23
31990 write(3, "\n", 1)                 = 1
31990 open("/tmp/sh-thd-106091305575", O_RDONLY) = 4
31990 close(3)                          = 0
31990 unlink("/tmp/sh-thd-106091305575") = 0
31990 fcntl(0, F_GETFD)                 = 0
31990 fcntl(0, F_DUPFD, 10)             = 10
31990 fcntl(0, F_GETFD)                 = 0
31990 fcntl(10, F_SETFD, FD_CLOEXEC)    = 0
31990 dup2(4, 0)                        = 0
31990 close(4)                          = 0
31990 dup2(10, 0)                       = 0
31990 fcntl(10, F_GETFD)                = 0x1 (flags FD_CLOEXEC)
31990 close(10)
...

Several steps are performed here:

1. Child process 31990 is spawned
2. The here string is written to /tmp/sh-thd-106091305575
3. /tmp/sh-thd-106091305575 is opened as read-only (FD 4)
4. /tmp/sh-thd-106091305575 is unlinked (so when FD 4 is closed it will be deleted)
5. stdin is duped to FD 10
6. FD 4 (the temp file containing our here string) is duped to FD 0 (stdin) This is where a process — in this case the bash subshell — would act on stdin
7. FD 4 is closed and FD 10 is duped back to FD 0

The surprising thing is that this ever happens in the input redirector example:

31929 write(1, "server1\nserver2\nserver3\n", 24) = 24

I suspect that bash has special behavior for this case. Will probably need to dig into the source for a definitive answer.