Hi:
I can recreate the situation:
Create a classic block add some text with table and a list and a paragraph and save the document. after saving exit the entire admin meaning close the tab.
Then reenter the tab and start writing to it.
Hit backspace somewhere in the text and see the cursor jumps to the start.
Select some text with the mouse and try to change it by hitting a new letter and it jumps the the start.
Shift+Enter does not work.