Why is Visual Studio Code not showing mypy errors when I have my package installed in editable mode in a venv?

I have a Python project which uses mypy for type checking. The root of my project contains a setup.py and the package folder rise, along with a virtual environment folder venv. Both my shells and VSCode are set to use this virtual environment.

Most of the time, this setup works great: VSCode runs mypy every time I save and marks the errors with squiggles and in the Problems window. But for some reason, if I install the rise package in editable mode using pip install -e ., type checking in VSCode never finds any type errors. This does not happen if I install it in non-editable mode with pip install ., and if I uninstall the editable-mode package and reload the VSCode window, it immediately starts working normally again.

If I go into the Python output in the Output window, I see:

> ./venv/bin/python ~/.vscode/extensions/ms-python.python-2022.18.2/pythonFiles/linter.py -m mypy --show-column-numbers --strict ./rise/action.py

[...output from pylint, which is also configured and is working normally, clipped]

##########Linting Output - mypy##########

Success: no issues found in 1 source file

Yet if I copy and paste the command it’s echoing into the terminal, there is in fact an error:

$ ./venv/bin/python ~/.vscode/extensions/ms-python.python-2022.18.2/pythonFiles/linter.py -m mypy --show-column-numbers --strict ./rise/action.py
rise/action.py:140:12: error: Incompatible return value type (got "None", expected "bool")  [return-value]

I’m stumped as to how to troubleshoot this further, and I haven’t been able to find any reference to this issue anywhere on the web.