Previously it was possible to create a module in the page editor that was required, locked, and did not have a default value. Meaning it was impossible to publish a page with that module, because the required field was not filled in, and had no way of being filled in by the content creator.
We're introducing new validation that checks for this issue. If found you'll receive a warning message when uploading your fields.json file, or publishing in the Design Manager. This is live right now.
In 90 days (December 14th) we will be changing this validation to an error. This will prevent developers from accidentally putting modules into this state. Existing modules that have this issue, will require being fixed before changes to them can be published. This change should impact only a very small number of existing modules.
To fix a field in this state, either provide a valid default value, or make the field not required.
If you produce a marketplace theme, it's encouraged that you review your modules to ensure they don't have this issue.