text input v3 preedit_string's cursor position isn't honored

The preedit_string event has the arguments to position the cursor, and I've configured my fcitx5 input method to put it at the beginning, so that the input method window doesn't move a lot during typing (i.e. when the preedit string length changes). But Qt still places the cursor at the end of the preedit string. This combining with QTBUG-121477 makes the input method window chases the cursor constantly.

I've tested Wireshark, Telegram and fcitx5-config-qt on Wayfire, and QT_WAYLAND_TEXT_INPUT_PROTOCOL=zwp_text_input_v3 dolphin on KDE. All reproduces this issue.