Custom attribute not always shown in Eclipse client

I have defined a custom attribute for the Details section of work item type epic.
Now the behaviour is as follows:
- Attribute field is always shown in Web UI, even if value is empty. So I am able to insert something
- Attribute field is not shown in Eclipse UI as long as the value is empty.
- As soon as the value is not empty (filled via the Web UI), the field is visible in the Eclipse UI, too.

I assume that I am doing something wrong but I cannot find out how to make the field always visible in the Eclipse UI, too.

Any help is greatly appreciated.

Thanks
- Thomas