GTK_IM_MODULE is not specified now. So maybe GTK X11 applications uses xim, which does not support showing candidate window under the cursor.
Note that GTK_IM_MODULE should not be set under GNOME wayland session because it now use Wayland input method protocol instead of GTK IM Module. We have to find out why GTK_IM_MODULE is not set even for X11 sessions.
GTK_IM_MODULE is not specified now. So maybe GTK X11 applications uses xim, which does not support showing candidate window under the cursor.
Note that GTK_IM_MODULE should not be set under GNOME wayland session because it now use Wayland input method protocol instead of GTK IM Module. We have to find out why GTK_IM_MODULE is not set even for X11 sessions.