Technically the extension is supposed to load after the dashboard is loaded so it has access to the information on the dashboard. Does your dashboard take a long time to load? Have you tried testing this on a simple dummy dashboard?
With tableau desktop the dashboard works fine, on tableau server the dashboard takes longer to load. I also noticed that the first time you load the dashboard the username sheet is empty but when you refresh then it starts showing data.
Edit: I tested the dashboard with the username sheet not hidden and I can see that the extension is loaded before the username sheet is populated. I think thats the problem. Anyway to ensure that this doesn't happen?