Note to self (and others interested in investigating the issue): things to check, suggested by an IME expert (thanks bochecha for the crash course on IMFs and IMEs on #gnomefr).
- does the GNOME platform snap include the IMF toolkit connector (ibus-gtk3)?
- does the sandbox allow the app to talk to the ibus daemon (flatpak has a mini-ibus that exposes the name org.freedesktop.portal.ibus on the session bus)