Seeing the exact same behavior on a different environment:

Mypy version used: mypy 1.9.0 (compiled: yes)
Mypy command-line flags: stubgen -m example
Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
plugins = "pydantic.mypy"

Python version used: 3.10.13

Also seeing this in my environment. I see it happening regardless if I define the class using either of these two

class A(BaseModel):
    foo: Optional[str] = "bar"

class B(BaseModel):
    foo: Optional[str] = Field(default="bar")

Version information:

$ python --version
3.8.19

$ python -m mypy --version
mypy 1.10.0 (compiled: yes)

$ pip freeze | grep pydantic  
pydantic==2.7.1
pydantic_core==2.18.2

Stubs are created using

$ stubgen -o module_dir module_dir/

and my pyproject.toml has the following:

[tool.mypy]
plugins = ["pydantic.mypy"]

I don't think it is related to pydantic because it also happens for basic classes like:

# .py
class A:
    foo: str = "bar"

... which generates these stubs:

# .pyi
class A:
    foo: str

mypy 1.10.0 (compiled: yes)
Python 3.12.2

Hello everyone! I stumbled upon the same issue. Does anyone know a workaround?