I'm currently experiencing this problem as well. Was there any resolution to this?
It would be easier to help you if you post your package workbook with sample data.
+1 -- even screenshots would be better than nothing. If we cannot see the setup, it is very difficult to provide help.
This is currently expected behavior on Tableau Server. The issue is fairly easily solved with good design and user education. As long as your fields are included in your tooltip, your users will always have the ability to view the value of the pane header that is not frozen in Server.
I hope this helps.