If you can see them in FSS UI, I guess you would be able to see them also on XPSExplorer, so this could be something related to cache. Did you try to discard cache issues at local level as I mentioned before? (emptying browser cache right before creating the Agent, using other browsers, etc)
Regarding your question on build, I recall the CR05 had a difference in build number only with non-Windows Policy Servers, so it should not be a problem, unless the engineer working in your case has spotted something new.
Also, keep in mind that you should use FSS UI or AdminUI, but not both at the same time, as this can create corruption. You could also run a XPSSweeper to see if there is a problem, just in case.
After creating a new agent, if you just close your browser and access again, are you able then to see it in the list?