Store not resubscribed when changed (as prop in component)

[theonlypwner]

Describe the bug
When a prop (whose value is a store) changes, Svelte does not unsubscribe from the old store and subscribe to the new store.

This seems related to #2014 and #2435, which were previously fixed.

Logs
See below.

To Reproduce
With local variables, it works as expected: https://svelte.dev/repl/f8dca40374cd499b9b2df84a900d908b?version=3.12.1

s1 subscribe
(swap)
s1 unsubscribe
s2 subscribe
(swap)
s2 unsubscribe
s1 subscribe

With a component prop, it does not: https://svelte.dev/repl/fa8f0e0b78f743e5b1410c87885cf277?version=3.12.1.

s1 subscribe
s1 subscribe
(swap)
s1 unsubscribe
s2 subscribe

If <h1>outer $s = {$s}</h1> is commented out, no subscribe/unsubscribe occurs when the button is clicked.

Extra unsubscribe statements appear in the console when editing the code when the old component is unmounted.

Expected behavior
The component should unsubscribe from the old store and subscribe to the new store.

Information about your Svelte project:

• Your browser and the version: Chrome 77.0
• Your operating system: Windows 10
• Svelte version: 3.12.1

Severity
It's not a big issue, but the current behavior seems surprising or counter-intuitive.

Additional context
None

[Conduitry]

Simpler repro:

Main.svelte

<script>
	import Foo from './Foo.svelte';
	import { writable } from 'svelte/store';
	let prop = writable(1);
	setTimeout(() => prop = writable(2), 1000);
</script>

<Foo {prop}/>

Foo.svelte

<script>
	export let prop;
</script>

{$prop}

After one second, this should switch from displaying 1 to 2, but does not. The $set handler

	$$self.$set = $$props => {
		if ('prop' in $$props) $$invalidate('prop', prop = $$props.prop);
	};

needs to also resubscribe to the store, like any assignment to prop would.

[Conduitry]

Duplicate of #3662.