The Boot Runner selection screen operates in front of the login window after startup but before logging in. FileVault encryption allows the system to be unlocked before boot, but passes through the credentials after startup. When FileVault is enabled, the Boot Runner screen never appears because the user credentials used to unlock FileVault pass through to automatically log in to the user account. The solution is to disable automatic login for FileVault.
To disable automatic login, open a Terminal window and enter:
sudo defaults write /Library/Preferences/com.apple.loginwindow DisableFDEAutoLogin -bool YES
To re-enable automatic login for FileVault, enter:
sudo defaults delete /Library/Preferences/com.apple.loginwindow DisableFDEAutoLogin
For more details, see Apple Support Article 202842