I don't use GNOME here (or more exactly, I am using a fork of GNOME2: MATE), but yes, this is typical of a shortcut interception by the window manager or related components; this could also happen (especially under Windows) with some gaming keyboard proprietary drivers: the final software (here, the viewer) does not even "see" the pressed keys arriving to its input queue, since they get intercepted by those components.
There is therefore nothing I can do at my level, short of changing the shortcut (like I did for CTRL ALT Fn ones, which are used by Linux to switch to virtual text consoles)... However, this cannot cover all the specific cases, and changing CTRL SHIFT U for something else won't guarantee it would solve the issue for
everyone.