Hi there,
I am using fcitx5 and don’t have this problem with Chinese character input in Whatsapp Web in Firefox (just tested).
sudo pacman -S fcitx5-im fcitx5-chinese-addons
should be all you need. fcitx5-im is a meta package, please accept all 4 elements contained therein.
Extensive info in the Arch Wiki.
And please disable/uninstall ibus before dealing with fcitx5. It seems to me that ibus is older and fcitx more current and better integrated in modern desktop environments (I am using KDE Plasma). I ditched ibus several years ago and never had a problem with fcitx.