Error disappears when just checking the erroneous file

I am encountering a weird error that when I check a whole module, an error is issued, but when I only check the affected file, the error disappears. [demo.zip](https://github.com/user-attachments/files/27795802/demo.zip)

```bash
(3.14) rscholz@Z890:/tmp$ pyright demo/
/tmp/demo/vectors.py
/tmp/demo/vectors.py:14:34 - error: Cannot assign to attribute "KNOWN_MEETS" for class "type[Vecs]"
  "Meet[VectorDomain]" is not assignable to "Meet[Self@Poset]"
    Type parameter "D@Meet" is covariant, but "VectorDomain" is not a subtype of "Self@Poset"
      "VectorDomain" is not assignable to "Poset" (reportAttributeAccessIssue)
1 error, 0 warnings, 0 informations
(3.14) rscholz@Z890:/tmp$ pyright demo/vectors.py
0 errors, 0 warnings, 0 informations
```

```python
# file demo/__init__.py
```

```python
# file demo/base.py
from enum import Enum
from typing import ClassVar, Protocol, Self

class DomainLike(Protocol): ...

class Domain(DomainLike, Protocol):
def __and__(self, other, /) -> Domain: ...

class Meet[D: DomainLike](Domain):
def __and__(self, other: DomainLike, /) -> Meet: ...

class VectorDomain(Domain):
def __and__(self, other: Self, /) -> Meet[VectorDomain]: ...

class Poset(Enum):
KNOWN_MEETS: ClassVar[tuple[tuple[Self, Meet[Self]], ...]]
```

```python
# file vectors.py
from .base import Poset, VectorDomain

class Empty(VectorDomain): ...
class Negative(VectorDomain): ...
class NonNegative(VectorDomain): ...

class Vecs(VectorDomain, Poset):
EMPTY = Empty() # bottom node
NEGATIVE = Negative() # xᵢ < 0
NONNEGATIVE = NonNegative() # xᵢ ≥ 0

Vecs.KNOWN_MEETS = ((Vecs.EMPTY, Vecs.NEGATIVE & Vecs.NONNEGATIVE),)
```

Start by extracting demo.zip and running pyright demo/ and pyright demo/vectors.py to reproduce the differing diagnostics. Inspect demo/base.py and demo/vectors.py, focusing on the generic types, Self annotations, and cross-file imports. Done means the whole-module and single-file checks report consistently for this example.

Written by the indexing model from the issue text.