?? and ??= behave differently when they shouldn't

TypeScript Version: 4.0.2, Nightly (2020-9-2)

Search Terms: Nullish coalescing operator Nullish never Coalescing never Nullish coalescing assignment

Code According to this post usage of ?? and ??= should be equivalent in the sample code below:

const values: { a?: string[] } = { };

delete values.a;
(values.a ?? (values.a = [])).push("hello"); // error
values.a = undefined;
(values.a ?? (values.a = [])).push("hello"); // error

delete values.a;
(values.a ??= []).push("hello");
values.a = undefined;
(values.a ??= []).push("hello");

Expected behavior: Examples with ?? and ??= should compile and behave similarly.

Actual behavior: Examples with ?? do not compile (Argument of type 'string' is not assignable to parameter of type 'never'.(2345))

Playground Link: link

Related Issues: Didn't find anything similar