When you edit a page layout, select a Related List with Properties (tool icon) and click the icon. Many of us not aware that Apply column information to other page layouts: at below if you not scroll down.
When you scroll down, you will see all other page layouts available and they are selected by default.
Conclusion: make sure none of this checkbox is selected, otherwise it will effect:
- Available fields, column order and Sort field asc/desc in other Page Layout selected will be overwritten
- Last Modified Date and By in other Page Layout selected will be updated.