Maps in item frames are only shown when in the login world. Example:

1. Login to server. Maps are shown.
2. Switch to another world. Maps are not shown.
3. Switch back to world in #1. Maps are shown.

In step 2 above, if you relog, maps will be shown in that world. But then switching to the original world above, maps will not be shown. So you can only see maps in whatever world you login to.