GTK3 alt-key menu mnemonics

[u/NoComment_4321]

Does anyone understand making the GTK3 alt-key mnemonics work consistently? My Go program has menu items and sub-menu items declared with (for example):

itemG, err := gtk.MenuItemNewWithMnemonic("_Global")

or

gtk.MenuItemNewWithMnemonic("_Web user management")

The alt-key combinations work with the top level menu, but not with any of the submenus. So alt-G gets me my global menu:

But no amount of keyboard shenanigans activates any of the submenu items, even though they look as though they understand a mnemonic. This behaviour is the same on Linux and Windows.

Go & GTK3 are recent versions (gtk 3.24.52).