Аdded a fourth sheet. Тhen began to edit the data on it (add and remove columns, enter the data) at the same time similar changes occurred on the third sheet.
That happens when you have the 3rd and 4th sheets selected at the same time. Can you double-check if that's what you were doing?
Check the sheet tabs at the bottom. The white tabs are the selected ones.
You are right. It's my fault. I did not know about this functionality and probably accidentally marked one more page. I'm sorry.