I have downloaded offline speech recognition for Chinese as well, so I don't think that is the problem. It seems that google has a number of different ways to recognize speech.
When I have the English keyboard enabled and I click the mic symbol at the top right of the keyboard, the top bar next to the microphone displays "speak now" and when I do, my words, for example right here, are instantly typed. This works the same way and equally well if I have the German, Russian or Hebrew keyboard enabled and I am speaking in German, Russian or Hebrew.
With Chinese however, something different happens. When I click the mic symbol, a window opens with a mic in a green circle at the center. To the left is a settings wheel which leads to the voice input section of my phone's settings. At the top a message is briefly displayed: "try saying something," which then changes to: "tap to speak." When I then speak in Chinese, the text is also quickly typed. However, this window, the one with the green circle, is different from the window that opens when I click the mic symbol in Pleco. When I click this, a window opens with the mic symbol in a blue circle. There is no settings wheel, and the text at the top says: "speak a word to look it up." At the bottom it says"普通话(中国大陆)," which btw., is also what I see when I look at the offline speech recognition section of my phone's settings. When I speak into this mic, my speech appears typed in the window, but it is not entered into Pleco's input box.
As I mentioned previously, my phone is a xiaomi A1, which is updated to the latest version of plain android 9, no MIUI.