Constant vs variable as index for none-checking List[Optional[...]]

Can you explain to me why there is this difference between using a constant and a variable as index for none-checking here?

from typing import List, Optional

my_list: List[Optional[float]] = []

if my_list[0] is not None:
    result_1: float = my_list[0]        # no error
    result_2: float = my_list[1]        # error: Incompatible types in assignment (expression has type "Optional[float]", variable has type "float")

id_0 = 0
if my_list[id_0] is not None:
    result_3: float = my_list[id_0]     # error: Incompatible types in assignment (expression has type "Optional[float]", variable has type "float")


for i in range(10):
    if my_list[i] is not None:
        a: float = my_list[i]           # error: Incompatible types in assignment (expression has type "Optional[float]", variable has type "float")

• Mypy version used: 0.800 0.941
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: python3.6
• Operating system and version: Ubuntu 18.04 in WSL2 (Windows 11)

The issue names no repository files or tests. Reproduce the examples with mypy 0.941, then trace the type narrowing behavior for indexed List[Optional[float]] expressions. Done means the differing constant, variable, and loop-index behavior is explained and, if confirmed as a bug, covered by an appropriate regression test.