Text cannot be deleted properly when using Microsoft Pinyin IME (and other CJK IMEs)

[BlueRain-debug]

When I use the Simplified Chinese Microsoft Pinyin IME with Microsoft Edit 1.2.1 on Windows Server Insider Preview build 26525, after I empty the input buffer, the first character I input is left in the content.

Repro:

1. Enable Microsoft Pinyin input method.
2. Run an instance of edit (wt or conhost).
3. Enter some letters in Kanji mode and delete them.