I am using TeXstudio 2.12.22 with Qt 5.9.5 R on Ubuntu 18.04.4 LTS, and I am typing in Japanese text using the Mozx IME.
Pressing backspace during the typesetting of an uncommited word behaves unexpectedly. [See https://sourceforge.net/p/texstudio/bugs/1256/ ]
I will walk you through an example. Let's assume that you have already typed in the commited text 女の into your editor, your cursor is at the rightmost position, waiting for your new input. Yo write 'kodomo' with your keyboard, and the IME writes you こども, waiting for you to commit to this text. If you hit ENTER, and accept this word, everything works as expected, you will have 女のこども on your screen.
However, let's assume that you are not happy with the word こども, and instead of commiting to it, you would like to change it to something else. Naturally, you start pressing backspace, and you expect that after hitting it 3 times, you will erase this word. However, this is not what happens. What happens is, that pressing it once leaves you with こど and pressing it once more leaves you with こ (as expected), but then, pressing it for the third time does nothing! Visually, you are still see on the screen こ, with the cursor blinking to the right of it!
Technically, what happens is that this last letter is in fact 'removed', as the next letter you type in will overwrite it. But this is neither indicated visually as the cursor is blinking to the right of this character, and also, because this is not what supposed to happen.
Typically, the user presses the backspace once more, to have this last charcter こ deleted, and this indeed happening. Unfortunately, if you start typing your desired word now, say 'hito', this extra backspace you just pressed kicks in, and deletes one more character from the left. So on the screen you see now 女ひと, waiting for to be committed (note the lost character の). So you commit to it (not even realizing the missing letter), and after commit you will get 女ひとひと, i.e. your word got duplicated.
This behaviour is somewhat similar to a bug reported earlier: https://sourceforge.net/p/texstudio/bugs/1256/
please report on github.com/texstudio-org/texstudio