Differed Protocol / runtime_checkable import inconsistently causes incompatible type error

Bug Report

An incompatible type error is raised for compatible types when Protocol & runtime_checkable are imported via a helper module (see # Option A: Via Helper), but not when they are imported within the module (see # Option B: Directly). Elsewhere in the codebase, importing via the helper module results in the expected behaviour.

Raises: error: Argument 1 to "handle" of "Interpreter" has incompatible type "ComposedAdapter"; expected "Adapter"

To Reproduce

# Option A: Via helper
from tickit.core.utils.compat.typing_compat import Protocol, runtime_checkable

# Option B: Directly
import sys

if sys.version_info >= (3, 8):
    from typing import Protocol, runtime_checkable
else:
    from typing_extensions import Protocol, runtime_checkable

@runtime_checkable
class Adapter(Protocol):
    interrupt: bool

    def __init__(self, device: Device) -> None:
        ...

    async def run_forever(self) -> None:
        ...

class ComposedAdapter:
    _interpreter: Interpreter
    _server: Server
    interrupt: bool = False

    def __init__(self, device: Device) -> None:
        assert isinstance(self._interpreter, Interpreter)
        assert isinstance(self._server, Server)
        self._device = device

    async def run_forever(self) -> None:
        async for interrupt in self._server.run_forever(
            lambda message: self._interpreter.handle(self, message) # Error raised here
        ):
            self.interrupt |= interrupt

Where tickit.utils.compat.typing_compat is implemented as:

import sys

if sys.version_info >= (3, 8):
    from typing import Protocol, runtime_checkable
else:
    from typing_extensions import Protocol, runtime_checkable

__all__ = ["Protocol", "runtime_checkable"]

Expected Behavior

No error is raised when importing via the helper module & behaviour is consistent across the codebase.

Actual Behavior

An error is raised when importing via the helper module.

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): ignore_missing_imports = True
• Python version used: 3.7.9, 3.9.5
• Operating system and version: Red Hat Enterprise Linux release 8.4 (Ootpa)

Reproduce the incompatible type error using the helper import from tickit.core.utils.compat.typing_compat and compare it with the direct typing or typing_extensions imports shown in the report. Trace how mypy handles Protocol and runtime_checkable through the helper module, then verify that both import styles produce consistent type-checking behavior for the Adapter and ComposedAdapter example.