I'm not sure what you mean. Perhaps you've seen one of the mill that was idle or canceled production. In that case for some reason the wicks disappear.
Anyway, this is not a bug and if what you mean is something else, please provide a screenshot. Secondly this is a graphical error, should it be legit, not a feature bug.
In the future, if you decide to post a bug report, check out this thread first and try the suggestions posted there:
Bug Resolve Suggestions and Bug Report Template
If none of the suggestions work, please create a bug report conform the correct template in the link I just provided.
This template is mandatory in order for us to help you to the best of our abilities.
- L