Unable to redirect stdout for background terraform process after it received input from named pipe

Question

I have a terraform file:

terraform {
  required_version = "1.3.5"
}


locals {
  a = "foo"
  b = "bar"
}

in a bash terminal, I can do:

$ echo "local.a" | terraform console
"foo"
$ echo "local.b" | terraform console
"bar"

Now what I'm trying to do is start a process running terraform console in the background and feed it commands.

This is what I've tried (following this answer https://serverfault.com/a/815253):

$ mkfifo /tmp/srv-input
$ tail -f /tmp/srv-input | terraform console >>output.txt 2>&1 &

this starts the background process correctly:

$ ps -ax | grep terraform
6030 pts/0    Sl     0:01 terraform console

if I then run:

$ echo "local.a" > /tmp/srv-input

the output file, output.txt is empty.

$ cat output.txt
$ 

If I run:

$ echo "local.c" > /tmp/srv-input # invalid input

the output file, output.txt contains the (expected) error:

$ cat output.txt
╷
│ Error: Reference to undeclared local value
│
│   on <console-input> line 1:
│   (source code not available)
│
│ A local value with the name "c" has not been declared. Did you mean "a"?
╵

[1]+  Exit 1                  tail -f /tmp/srv-input | terraform console >> output.txt 2>&1

---

Why is only the stderr being redirected to the log file, but not stdout?