It might affect filtered content only.

Here is some simple PoC CSV data (two rows):

  1. Open those two rows in cell selection mode (CSV) inside EmEditor.
  2. Try to edit any cell – it will work as intended/expected.
  3. Now filter out the string “col_333” in column 3 (which in turn will make the second row disappear from view).
  4. Then try to edit any of the cells on the remaining row.
  5. You will now see the described behavior.

(and no, I don’t have any plugins, and neither any active macros when this happens)