This annoys me for several years already.

Steps to reproduce:
1) Start ibus-daemon.
2) Start a Qt application (the same happens for me in Viber, telegram-desktop, Nheko).
3) Quit ibus-daemon.

While other apps handle this properly, typing in Qt apps becomes broken until they're relauched. Most of time typing just doesn't work: pressing keys produces no letters; if trying to retype some letter actively, it may eventually pass, sometimes even several letters at time. This applies not only to alphanumeric keys, but also to control keys like Home: they're usually not handled.