Password input(dead key) broken with QT_IM_MODULE=ibus

If I execute the following program:

#include <QApplication>
#include <QLineEdit>
#include <iostream>

 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);

     QLineEdit hello;
     hello.setEchoMode(QLineEdit::Password);

     hello.resize(100, 30);

     hello.show();
     app.exec();
     std::cout << hello.text().toStdString() << std::endl;
 }

and input an á in the password dialog using my dead key and QT_IM_MODULE=ibus, the program prints a, as if there were no accent. If I try again with QT_IM_MODULE=xim, the output is á as expected.

Personally, I am on Ubuntu 18.04 with Qt 5.9, but an Arch Linux user with Qt 5.12 confirms the issue is still present: https://github.com/spesmilo/electrum/issues/5038#issuecomment-458743135

It affects all Ubuntu users with dead keys since GNOME seems to set QT_IM_MODULE=ibus.

Please tell me how I can help.