Unsolved Why I can not switch to chinese input when editting QPlainTextEdit?

[Kobe Green]

Hello, I created a PySide2 UI program.And There was a QPlainTextEdit in it.When I load json file which contained Chinese Characters,all worked well.But when I click the QPlainTextEdit and tried to switch chinese input.The input method remained default English.The input method switched to Chinese in other windows so it is normal.Can someone tell me WHY?