When editing a page, content is often copied to the top, tables are copied and mangled

When editing a page, particularly when copying and pasting, content frequently gets copied to random places, often to the top of the page.

When I return to edit pages, I often notice that my tables have taken on columns from other tables and the cell content is a mixture of what was already in the table and what has been copied from another table.

It seems fairly random but is extremely frequent. Just by doing a lot of editing, you can reproduce.