Registering custom widgets for Qt Designer fails without error

I'm trying to register my custom widgets for use in Qt Designer using this guide:

https://doc-snapshots.qt.io/qtforpython-dev/tutorials/basictutorial/uifiles.html#custom-widgets-in-qt-designer

The basic example works, but as soon as I try anything practical (with external dependencies) the registration fails.

There is no warning or error message. The widget will simply not show up in Qt Designer.

Some concrete examples that cause the registration to fail include:

• using relative imports in registerXYZ.py

from .custom_widget import CustomWidget

• importing numpy in registerXYZ.py or anywhere else down the import tree
• importing any package that is installed in editable mode