Hey Ray!
Due to how snap is architected, each snap has to include the libraries that it needs, and input method is one of them.
Another quick check on electronic-wechat, there is no immodule for gtk3 but only gtk2:
$ find /snap/electronic-wechat/ -name im-fcitx.so
/snap/electronic-wechat/7/usr/lib/x86_64-linux-gnu/gtk-2.0/2.10.0/immodules/im-fcitx.so
Not sure if this is the reason why you can’t use fcitx or caused by other reasons, I don’t have time to look deeper.
Gnome apps such as gedit makes use of the gnome runtime snap and connect to it through content interface, like this:
https://gitlab.gnome.org/GNOME/gedit/commit/56ff93fe359cd7a272c70f7ad5d870a027487dc5
So gedit doesn’t include IM libraries and just rely on the gnome runtimes such as gnome-3-28-1804.
I think desktop app snap developers should be made more well aware of these pitfalls.