You should probably include os, browser and language when reporting an ime issue. In this case I don’t see any issue in Safari but do in Chrome (on mac and windows) where the ime composition seems to end on the first key stroke. I tried pinyin and hiragana so probably not language specific.
sorry ， forgot it
os : macOs monterey 12.0.1
browser: chrome 99.0.4844.74
This is a result of limitations in the way browsers handle composition — if the DOM around the composition, or the DOM selection, is changed in any way after the composition starts, the browser will abort the composition. Unfortunately, cross-block insertion will sometimes cause the browser to make a mess out of the surrounding DOM, which ProseMirror will then correct, causing the composition to abort. I’m not currently aware of a reasonable way to solve this.