GNotification notifications are ignored
Here notifs.c is a simple test program which reproduces an issue that a GNotification notification is ignored by GNOME Shell. The first line contains a command line to compile & run the program. Using libnotify directly doesn't exhibit the issue. I can reproduce it with 3.28.x, but also with gnome-shell-3.29.90-2.fc29.x86_64. I get no issue using GNotification with MATE, it just works.
I understood from the description of https://developer.gnome.org/gio/2.56/GNotification.html that this is the preferred way of showing application notifications, without a need to depend on the libnotify.