Option key accents add an extra character on macOS/Webengine+Japanese input

Edit: beta 3 has partially solved this issue, but the issue still exists in the Japanese keyboard layout - please see my latest comment.

To reproduce, using the markdowneditor sample app, add an <input> element, then in the input on the right, type

option+e then e

Instead of just an é, it inserts eé. Same for other dead key combinations option+u, u - it yields uü instead of ü.

Problem does not exist in Qt 5.11.