def typed with Union passed twice is wrongly reported "Incompatible return value type"

mypy errantly cites error got "Tuple[Optional[str], Optional[str]]", expected "Union[Tuple[str, str], Tuple[None, None]]". Seems to mix up the Union and Optional.

First, given file test1.py that passes mypy

from typing import Union, Tuple

def def1(var: bool) -> Union[Tuple[str, str], Tuple[None, None]]:
    a, b = "", ""
    if var:
        return a, b
    return None, None

Run command

python -m mypy test1.py

(test1.py passes).

Then add a call to another function with the same type declaration.

Given file test2.py

from typing import Union, Tuple

def def1(var: bool) -> Union[Tuple[str, str], Tuple[None, None]]:
    if var:
       return "", ""
    return None, None

def def2(var: bool) -> Union[Tuple[str, str], Tuple[None, None]]:
    a, b = def1(var)
    if var:
        return a, b
    return None, None

The same mypy command fails test2.py

python -m mypy test2.py
test2.py:8: error: Incompatible return value type (got "Tuple[Optional[str], Optional[str]]", expected "Union[Tuple[str, str], Tuple[None, None]]")
Found 1 error in 1 file (checked 1 source file)

Environment

• Mypy version used: mypy 0.812
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.5
• Operating system and version: Windows 10 Pro

Run python -m mypy test2.py and compare it with test1.py, focusing on the type-checking path for unpacking the Union[Tuple[str, str], Tuple[None, None]] result in def2. Done means test2.py no longer reports the incorrect incompatible-return-value error while test1.py remains passing.