False positive `[operator]` on union of dicts with heterogeneous value types

Bug Report

If I take the union of two dicts with different value types, I cannot assign the result to a variable that is annotated with the resulting type. Doing so causes an error. Assigning to an non-annotated variable does not cause an error, which seems like the correct behavior.

Do I really need the annotation on d5 below? No. But I have a non-toy example of this in a code base where d5 is a type annotated attribute on an object, which results in the same error.

d1: dict[str, str] = {}
d2: dict[str, str | None] = {}

d3 = d1 | d2
reveal_type(d3)  # builtins.dict[builtins.str, Union[builtins.str, None]], aka dict[str, str | None]

# This is fine
d4: dict[str, str | None] = d3  # aka d1 | d2

# This isn't?
d5: dict[str, str | None] = d1 | d2

To Reproduce

See: https://mypy-play.net/?mypy=master&python=3.13&flags=show-error-codes&gist=2643cb3ff1ea956a1c4ed0869140d46d

Expected Behavior

I expect d5: dict[str, str | None] = d1 | d2 to be legal.

Actual Behavior

error: Unsupported operand types for | ("dict[str, str]" and "dict[str, str | None]") [operator].

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

Start with the minimal reproduction in the issue or its linked mypy-play example, using the reported Python 3.12 and mypy 1.13.0 context. Compare the annotated direct union with the intermediate variable case, then verify that the direct assignment no longer emits the [operator] error while the revealed type remains dict[str, str | None].