cat in process substitution hangs: what is it waiting for?

Question

I have this piece of code using Process Substitution.

When I run it, it works fine, but it does not exit - just hangs there waiting for input.

Why and how to overcome?

prepro() { 
        in=$(< <(cat) )         # capture input
        echo -e "$in"
}

echo -e "some words" > >(prepro)
echo "FINISHED"

Instead if I do this, it exits immediately.

echo -e "$in" >>"test.log"

What am I missing here?

CLARIFICATION: BASH 4.4.19(1)-release

On my pc, I see a difference between running echo -e "$in" and running echo -e "$in" >>"test.log".

The former does not give me a prompt (but I can run commands normally). I get:

user@server:~$ ./test.sh
FINISHED
user@server:~$ some words
█

I have to press ENTER to get my prompt back.

The latter gives me the prompt normally.

user@server:~$ ./test.sh
FINISHED
user@server:~$ █

I am curious if I am doing something wrong here.