IBus candidate box is invisible when 'Screen Keyboard' is on

Affected version

Gnome Shell 3.36.3 (Arch Linux) running in both Xorg and Wayland

Bug summary

IBus candidate box is invisible when 'Screen Keyboard' is on

Steps to reproduce

1. Install an input method (like ibus-libpinyin) then restart ibus-deamon: ibus-deamon -rdx;
2. Add it in Setting -> Region & Language then press Super+Space to switch input source;
3. Turn Screen Keyboard on and type anything in any text entry.

What happened

The preedit text is shown but the candidate box is hidden. By the way, almost everything is default here.

What did you expect to happen

Show candidate box when typing

Relevant logs, screenshots, screencasts etc.

Screencast