To reproduce on WP8.1:
1. Obtain a windows phone with the Chinese Simplified QWERTY Keyboard installed. This is harder than it looks..
2. Create a new unity project
3. Add a uGUI input field
4. Build & run for WP8.1
5. Press the input field, a keyboard appears
6. Select the Chinese language
7. Enter "wo". Notice how no text suggestions appear

To reproduce on WSA:
1. On Windows add Chinese language for keyboard input.
2. Create a new unity project.
3. Add a uGUI input field.
4. Build to WSA 8.1 or WSA Universal 10.
5. Launch project on PC and try to input Chinese characters in the input field. Observe there are no suggestions as doing the same on non-Unity application.

This works fine if deploying as WP8 app (suggestions appear at the top of the keyboard)

---
Won't fix:
There is no API to do this universally on WSA 8.1 and on WP8.1. It crashes due to a bug on Microsoft side, which they only fixed on WP10 (and won't fix on 8.1).
This is in progress of being fixed on UWP.