Hello.
Thank you for contacting us and reporting this issue.
We have reproduced your problem, and confirmed that when Xcode server is activated NoMachine shows _xcsbuildd desktop instead of the user that is really logged in. We are working on a solution for this problem.
We have also found a workaround that should work for the current NoMachine version:
When server is running and Xcode service is started you can do the user switch to other account which is currently inactive (without logging off), then when you switch back to main account, the NoMachine should now properly recognize it as active.