What version of Tableau Desktop/Server are you using?
About Tableau says: 8.0.0
I have just discovered exactly the same problem publishing my workbook to Tableau Server.
I also have Version 8 Desktop and Server.
My shape is an Up/Down, same as Fred's, but mine is just a simple calculated field sum(a) - sum(b)
I just had this very same issue in 9.0.5. which was frustrating. Refreshing the data source was causing it.
After further investigation I noticed these were on Calcs that were not materialized. When I materialized the calcs by dragging them into the data pane the issue went away.
The issue is happening because of the formula being directly used in the Shape Shelf.
Create a calculated field first and then drag it onto the Shape for them to be consistent when you publish to the Server.