Assignment with a callable: inconsistent kwarg handling

Bug(?) Report

Two problems:

1. reveal_type(a) and reveal_type(b) yield identical answers while at the same time these 2 types behave differently when used in lines 14 and 15.
2. mypy gives no error for line 14 (aa: F = a) while (to my understanding) it is within normal mypy capabilities to catch the kwarg mismatch. In other words, we would have a runtime error with aa(x=1) while mypy stays happy.

To Reproduce

https://mypy-play.net/?mypy=master&python=3.10&gist=090ff9d41c1b1e3012bf96720b0900d4

from typing import *

class F(Protocol):
    def __call__(self, x: int) -> int: ...

a = lambda y: 0

def b(y: Any) -> int:
    return 0

reveal_type(a)
reveal_type(b)

aa: F = a
bb: F = b

Expected Behavior
1, reveal_type should reveal enough details so that mypy behavior appears consistent.
2. For both the lines 14 and 15 mypy should discover the type incompatibility (due to argument name mismatch).

The output

/home/toton/tmp/f.py:11: note: Revealed type is "def (y: Any) -> builtins.int"
/home/toton/tmp/f.py:12: note: Revealed type is "def (y: Any) -> builtins.int"
/home/toton/tmp/f.py:15: error: Incompatible types in assignment (expression has type "Callable[[Any], int]", variable has type "F")  [assignment]
/home/toton/tmp/f.py:15: note: "F.__call__" has type "Callable[[Arg(int, 'x')], int]"

Your Environment

$ venv/bin/python --version
Python 3.9.2
$ venv/bin/mypy --version
mypy 1.0.0+dev.a4da89e0543ee0d213b30216c9e15a5d00fcd578 (compiled: no)
$ venv/bin/mypy ~/tmp/f.py
.....

Start with the mypy-play reproduction and the callable assignment and reveal_type cases shown in the issue. Compare the behavior for the lambda and annotated function, then verify that both incompatible keyword-argument assignments are diagnosed and that revealed types explain the difference.