This bug has been working for several dozen versions, and is still not fixed.
It will be difficult to explain, but in the video we see:
-When a player writes something on (chat, sign, command block, book, etc.), then everything is fine.
-When the player wants to move the letter icon (green arrow), it is not displayed correctly, and it is impossible to understand which part of the text you have moved to.
-Sometimes it even leads to some words being inserted into the text, or the whole text is erased.

This problem is related to the writing function itself inside Minecraft.
Steps to reproduce:

In new command block, enter the following text using the regular Android on-screen keyboard: "Here is some example text". Alternatively, you can enter the text in a book and quill or the chat window instead of a command block.
Move the cursor after the word "Here" by touching the end of the word.
Enter the text "xxx".
Expected Result:
The "xxx" is placed at the end of the first word.

Observed Result:
The "xxx" is placed at the end of the text, and the word "is" is deleted.