trap EXIT in piped subshell not triggered during wait

[Ilya Basin]

Hi!
I've got strange behavior. Here's my script:

    #!/bin/bash
    {
        trap '
            echo "in trap EXIT">&2
        ' EXIT
        sleep 4 &
        echo 'sleep 2'>&2
        sleep 2
        echo 'wait $!'>&2
        wait $!
        echo 'exit'>&2
        exit
    } | cat

If I press Ctrl-C during wait, the trap isn't triggered.
If I replace curly brackets with round brackets, it works. What's the
difference?