This is a problem related to Apparmor. See dmesg or /var/log/syslog.
I fixed it by changing /etc/apparmor.d/home.tor-browser.firefox. I added a line:
@{HOME}/.config/ibus/bus/* r,
in home.tor-browser.firefox.
I’ve also found Fcitx5-mozc works well without this type of apparmor problem. Switching to fcitx5 is an alternative solution.