When you select and copy a whole table in the editor and paste it into a cell of another table, the target table will be altered and populated with the content of the source table instead of just having the copied table present in the target cell.

Workaround
Place the cursor into the target cell, create a table in that cell with identical number of rows and columns as the source table, then copy and paste the table, standing in the top-first cell of the newly created table. This action will populate it with the data from the source table.