Where does astroid change its behavior for pylint checks of relative imports?

On a Linux environment, I have the following three files in a folder named utils:

1. An empty file __init__.py

2. A file named mymain .py with the following content:

    from mytool import foo
   

3. A file mytool.py with the following content:

    from subprocess import check_output  
   
    def foo():
        print("Hello World")
   

Now I create a virtual environment with the following setup:

astroid==2.4.2
isort==5.7.0
lazy-object-proxy==1.4.3
packaging==20.9
pylint==2.6.0
pyparsing==2.4.7
six==1.15.0
toml==0.10.2
typed-ast==1.4.2
wrapt==1.12.1

Then I run the pylint check on that folder

pylint utils

or

pylint --disable=C,R,W utils  # only the error is of interest

it should return a score of 10.

When you now upgrade the version of astroid to 2.5.0 and run the check again, you get the error:

************* Module utils.mymain
utils/mymain.py:2:0: E0401: Unable to import 'mytool' (import-error)

Is the changed behavior because of the deprecated importlib methods, as pointed out HERE?

Interesting fact

If you remove the unused import statement in the file mytool.py, you also get the same import error even with astroid version 2.4.2

Why; How can an unused import possibly affect the outcome of a pylint check?

Hint: I have two questions.

from Where does astroid change its behavior for pylint checks of relative imports?