Backspace key on preedit doesn't work on the search box at Activities Overview

IBus can be activated on the search box at the Activities Overview though, the characters in the preedit can be deleted by Backspace. the key events seems not forwarding to IBus.

I'm facing this issue with: gnome-shell-3.27.92-1.fc28.x86_64 gjs-1.51.91-1.fc28.x86_64 ibus-1.5.18-1.fc28.x86_64 ibus-kkc-1.5.22-9.fc28.x86_64