I cannot reproduce your issue. To enable XDMCP, I think you enable GDM login screen at console and a process of ibus-daemon is running owned by gdm user. Xephyr process shows GDM login screen remotely and it runs another ibus-daemon is running owned by gdm user. Both the GDM login screen and ibus-daemon is terminated with the timeout. If I log into the GNOME session via GDM login screen, ibus-daemon runs owned by the session user but the both the session and ibus-daemon is terminated with the timeout.