I have also seen this problem. M1 Air - 11.3.1 Intel MBP 2019 - 11.3.1 TextEdit 1.16 (365.2) if you paste text into a text format document and the text is enough to cause the window to scroll, subsequent cursor motions or typing cause the text in the window to jump around. A small amount of text that does not autoscroll the window does not have this affect. It looks like the cursor insert or display position is lost when the window autoscrolls to accommodate the text. (manual scroll or Cmd-Up and Cmd-Dn) If you scroll to the top of the file and back to the bottom again, then the cursor position is reset and editing happens normally.