Why does a Bash script with select exit without input when reading from stdin?

I'm working on a Bash script that optionally accepts input from stdin and then presents the user with a selection menu using select. The issue arises when the script is provided data via stdin—the select menu displays but exits immediately without accepting any input. It works fine when no stdin data is provided. Here is a minimal example: #!/usr/bin/env bash if [[ -p /dev/stdin && ${#bar[@]} -eq 0 ]]; then while IFS= read -r foo; do bar+=("$foo") done