Substring Expansion doesn't respect IFS when using in Heredoc

[Aloxaf Yin]

Hello, I encountered something strange when using bash. It seems like a bug
so someone suggested that I ask here.

Here is the test code.
    declare -a arr=(12 34 56 78 90)

    IFS=','

    cat <<<"${arr[*]:0}"

    cat <<EOF
    ${arr[*]:0}
    ${arr[*]}
    EOF

The result is as follows. As you can see the comma is missing from the
second line.

    12,34,56,78,90
    12 34 56 78 90
    12,34,56,78,90

bash: 5.1.16(1)-release
OS: Arch Linux