I have a repo with this tree:

directory_root

    packageA
        setup.cfg
        module1.py
        module2.py

    packageB
        module3.py

With packageA/module1.py having the following contents:

import module2
import packageB
import numpy
import os

and packageA/setup.cfg having the following contents:

[isort]
known_first_party = bla

If I run isort with:
python -m isort --verbose --check-only packageA/module1.py packageA/module2.py packageB/module3.py
I get the following output:

                 _                 _
                (_) ___  ___  _ __| |_
                | |/ _/ / _ \/ '__  _/
                | |\__ \/\_\/| |  | |_
                |_|\___/\___/\_/   \_/

      isort your imports, so you don't have to.

                    VERSION 5.11.2

else-type place_module for module2 returned FIRSTPARTY
else-type place_module for packageB returned THIRDPARTY
else-type place_module for numpy returned THIRDPARTY
else-type place_module for os returned STDLIB
ERROR: /directory_root/packageA/module1.py Imports are incorrectly sorted and/or formatted.
SUCCESS: /directory_root/packageA/module2.py Everything Looks Good!
SUCCESS: /directory_root/packageB/module3.py Everything Looks Good!

which is what I expected. So if I run isort without --check-only option, the original packageA/module1.py file is reformatted to:

import os

import numpy
import packageB

import module2

However, I want to start having support for multiple config files in a single isort run, so I have to use the --resolve-all-configs argument:
python -m isort --verbose --resolve-all-configs --check-only packageA/module1.py packageA/module2.py packageB/module3.py
which gives me the following output:

                 _                 _
                (_) ___  ___  _ __| |_
                | |/ _/ / _ \/ '__  _/
                | |\__ \/\_\/| |  | |_
                |_|\___/\___/\_/   \_/

      isort your imports, so you don't have to.

                    VERSION 5.11.2

./packageA/setup.cfg used for file packageA/module1.py
ERROR: /directory_root/packageA/module1.py Imports are incorrectly sorted and/or formatted.
./packageA/setup.cfg used for file packageA/module2.py
default used for file packageB/module3.py

First Question:
Why is the verbose debug information different when using the --resolve-all-configs argument?
The else-type place_module for <some_import> returned <SECTION_NAME> messages are gone, as well as the SUCCESS: <some_file> Everything Looks Good! message.

Now, let's see the reformatting result when I run with argument --resolve-all-configs and without --check-only option:

import os

import module2
import numpy

import packageB

To me this is quite unexpected, packageB went from THIRDPARTY to FIRSTPARTY and module2 went from FIRSTPARTY to THIRDPARTY.

Second Question:
(related to known_first_party inconsistencies!)
Why is the sorting result different when using or not the the --resolve-all-configs argument, if the config .cfg file used is the same?
If the same packageA/setup.cfg is used, I would expect the same sorting result.

Furthermore, I noticed using --resolve-all-configs argument makes isort much slower, taking a couple seconds for such a simple repo.
However, I also noticed that adding more folders/packages makes isort even slower, even if the request is exactly the same (sorting module1/2/3 only in package A and B).
For example, if I added 20 more packages (packageC, packageD, ..., packageZ), with plenty of folders inside each one, the isort run with the --resolve-all-configs argument took almost 20 seconds, even if only 3 modules in 2 packages are being reformatted.

Third Question:
Why does isort with --resolve-all-configs argument become slower when adding more folders/packages, even if it is still sorting the same number of files?
Shouldn't the search for the closest config file happen only for the files that are requested to sort, thus making isort run time only dependent on the number of files to check/reformat instead of the repo size?

Finally, I went back to testing the isort standard behavior when using only a single config file (without the --resolve-all-configs argument).
I changed the packageA/setup.cfg file by removing the known_first_party entry, thus making it empty, which I expect to make it resort to default configuration:

[isort]

If I run isort again with:
python -m isort --verbose packageA/module1.py packageA/module2.py packageB/module3.py
I get the following output:

                 _                 _
                (_) ___  ___  _ __| |_
                | |/ _/ / _ \/ '__  _/
                | |\__ \/\_\/| |  | |_
                |_|\___/\___/\_/   \_/

      isort your imports, so you don't have to.

                    VERSION 5.11.2

else-type place_module for module2 returned THIRDPARTY
else-type place_module for packageB returned FIRSTPARTY
else-type place_module for numpy returned THIRDPARTY
else-type place_module for os returned STDLIB
Fixing /directory_root/packageA/module1.py

which reformats the original file in this way:

import os

import module2
import numpy

import packageB

Again, compared to the original configuration, packageB went from THIRDPARTY to FIRSTPARTY and module2 went from FIRSTPARTY to THIRDPARTY.

Fourth (and Final) Question:
(related to known_first_party inconsistencies!)
Why do both packageB and module2 change sections if I have a default configuration vs known_first_party = bla, where bla was not even a package existing in my repo?

For module1 formatting, I expected always to have module2 as FIRSTPARTY because it's part of the same python package and lives in the same directory, and packageB as THIRDPARTY because it's a separate package from packageA where module1 lives.
And why setting known_first_party to some specific name would have an impact? Does it replace the default FIRST_PARTY, or extend?

Additionally and probably the most important question in this issue, why does --resolve-all-configs argument change the sorting result if exactly the same config file is applied?