Assigning to store whose value is an object is broken in version 3.25+

[Reikooters]

Describe the bug
When assigning a value to a store whose value is an object, the whole store value now gets overwritten beginning from 3.25 and above. Working in 3.24. See more details with example in "to reproduce" below. Also broken in 3.26.

To Reproduce
See below repl links, same code but in version 3.24 and 3.25.

The example creates a store which is an object containing two strings. First it logs the store's value to the console.
Then it overwrites the value of one of the strings in the object, and logs the store's value to the console again.
In version 3.24, the object got updated correctly. In version 3.25, the entire object got overwritten.

https://svelte.dev/repl/d9a6917ded77445d97af5ff826890e93?version=3.24.1
https://svelte.dev/repl/d9a6917ded77445d97af5ff826890e93?version=3.25.1

Version 3.24 gives the following output - string2 has been overwritten which is correct.
▶
Object { string1: "abc" ,string2: "" }
▶
Object { string1: "abc" ,string2: "def" }

Version 3.25 gives the following output - instead of updating the value of string2 in the object, the entire object got overwritten with the string.

▶
Object { string1: "abc" ,string2: "" }
"def"

Expected behavior
Expected behaviour is as per the result using Svelte version 3.24.

Severity
Blocking an upgrade to newer Svelte version. In my application I'm using stores for accessing global data, e.g. API Urls, tokens, user preferences etc used throughout the application. This change breaks the whole app.

[Conduitry]

Duplicate of #5412 - this is already fixed on master and will be released soon.