The select initial value is different to the placeholder's empty string, that's why it is not selected by default. Make sure to bind the select value to a variable that match the placeholder at initialization like this:

<script>
    let placeholder = 'hello world';
    let options = [{
        label: "Option 1",
        value: "option1"
    }, {
        label: "Option 2",
        value: "option2"
    }, {
        label: "Option 3",
        value: "option3"
    }];
    let selectedValue = "";
</script>

<select bind:value={selectedValue}>    
    {#if placeholder}
        <option value="" disabled selected>{placeholder}</option>
    {/if}
    {#each options as option}
        <option value={option.value}>
            {option.label || option.text}
        </option>
    {/each}
</select>

Working REPL: https://svelte.dev/repl/e1c7d9b804414f2d888b96b3e812a5c7?version=3.43.0