FormAction items have unusual iteration behaviour

Describe the bug

I'm using Form Actions to return a revised array of items in order to dynamically add form items when JS is not available.

<script>
	// ...
	export let form;

	let items = []
	items[0] = ['pets', '']
	$: {
		if (form?.items?.length > 0) {
			items = form.items
		}
	}

	const add = (e) => {
		return // remove this `return` for a 'working' loop
		e.preventDefault()
		items.push(['pets', ''])
		items = items
	}
</script>

<form method="POST" action="?/submit" use:enhance>
	{#each items as [key, value], i}
		<label>
			Pet name #{i + 1}
			<input name="pets" value={value} />
		</label><br />
	{/each}
	<button
		formaction="?/add"
		on:click={add}
	>Add item</button>
        <!-- ... -->
</form>

Where the "Add item" button goes to the back-end action and returns an updated array of items.

I expect the named action to return an updated array, and it does, but they're not rendered in the correct order, nor do some of the names appear after adding clicking 'Add item' a few times with entered data.

Reproduction

https://www.sveltelab.dev/e7a4t8onvtul9th

• Observe 'debug' text displaying proper array items, yet they aren't reflected in the #each loop
• Enabling JS to perform .push() client-side works as intended, comment out Line 15 to see

Logs

No response

System Info

System:
    OS: macOS 12.6.6
    CPU: (8) x64 Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
    Memory: 224.02 MB / 16.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.16.0 - ~/.nvm/versions/node/v18.16.0/bin/node
    Yarn: 1.22.19 - ~/.nvm/versions/node/v18.16.0/bin/yarn
    npm: 9.6.7 - ~/.nvm/versions/node/v18.16.0/bin/npm
  Browsers:
    Brave Browser: 114.1.52.126
    Chrome: 114.0.5735.133
    Safari: 16.5
  npmPackages:
    @sveltejs/adapter-auto: ^2.1.0 => 2.1.0 
    @sveltejs/kit: ^1.20.4 => 1.20.4 
    @sveltejs/vite-plugin-svelte: ^2.3.0 => 2.3.0 
    svelte: ^3.59.2 => 3.59.2 
    vite: ^4.3.8 => 4.3.8

Severity

serious, but I can work around it

Additional Information

No response

Start by reproducing the behavior in the linked SvelteLab example, focusing on the Form Actions response and the subsequent {#each} rendering. Trace how the returned items reach the client and compare their order and names after repeated additions; done means server-returned items render in the correct order with all names preserved.