[Python] Why does mypy error on returning list[str, int] + list[int]?

From mypy's point of view l1 + l2 is OK. But returning l1 + l2 isn't OK. Why?

I'm using Python 3.11 and mypy 1.16.

def test() -> list[str | int]:
l1: list[str | int]
l2: list[int]
l1 + l2 # OK
return l1 + l2 # error: Unsupported operand types for + ("list[str | int]" and "list[int]") [operator]

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