'No module named' in one script, but not another at the same level

Question

I'm still a novice at Python, but I've learned quite a few things. However, I've finally hit a brick wall that I can't solve on my own.

I'm trying to run an application that utilizes a shared module that gets used between two applications. I've also been writing some tests in a separate folder at the same level. When I attempt to run app1 from the command line, it's throwing a 'no module named' error, referencing the shared module in the stack trace. When I run pytest from the same location the tests that reference the shared module it executes without this error.

I've made sure that the relevant folders contain the __init__.py file. I've also made sure I'm running the python3 ./App1/server.py command from the appropriate location (proj1). My development uses virtualenv, and I've made sure it's active.

Folder structure used:

proj1
    -- app1
        -- __init__.py
        -- app
            -- __init__.py
            -- some_flask_controller.py
        -- server.py
    -- app2
        -- __init__.py
        -- No other relevant details here
    -- shared
        -- __init__.py
        -- some_file.py
    -- tests
        -- __init__.py
        -- shared_tests
            -- __init__.py
            -- test_some_file.py

Stack when attempting to run python3 ./App1/server.py:

Traceback (most recent call last):
  File ".\App1\app.py", line 1, in <module>
    from app import app
  File "\Proj1\App1\app\__init__.py", line 10, in <module>
    from app.some_flask_controller import mod_requests as request_module
  File "\Proj1\App1\app\some_flask_controller.py", line 5, in <module>
    from shared.some_file import some_class
ModuleNotFoundError: No module named 'shared'

I'm expecting either pytest and python3 ./App1/server.py to return this error, or both work. However, only pytest is executing normally. Is there a reason for this?