It turns out the information I've seen is incorrect. You need to include the path to your imported scripts in the pathex portion of the spec file (or use --paths when building without a spec file.

a = Analysis(
    ['c:\\Users\\name\\python_projects\\projectname\\src\\projectname\\main_script.py'],
    pathex=['c:\\Users\\name\\python_projects\\projectname\\.venv\\Lib\\site-packages', 'c:\\Users\\name\\python_projects\\projectname\\src\\projectname'],
    binaries=[],
    datas=[],
    hiddenimports=[],
    hookspath=[],
    hooksconfig={},
    runtime_hooks=[],
    excludes=[],
    noarchive=False,
    optimize=0,
)