A custom field based on the affected versions is given a default value while in the previous version of the app this field was empty (unknown)

I have just make a quick test. This is the build-in behavior of "Version Picker (single)" fields. It displays "Unknown". Even if you disable the add-on you will still see "Unknown" as selected by default. *But it will not set value of the field to "Unknown" it will be empty. *

You are correct, that’s why I deleted my comment, so you can ignore it.

Thanks for the quick reply.