Thank you very much for the sample GraphML file.
There are four ways to edit/specify the label text for a row in a pool node in yEd - and all four work for me with your diagram.
Step 1: Select the row for which you want to enter/change the label text.
1.1 Place the mouse cursor inside the row close to its left border.
1.2 Click the left mouse button to select the row.
Step 2: Edit the label.
press F2 and enter some text in the inline label editor, or
click the right mouse button to open the row context menu, choose "Edit Label", and enter some text in the inline label editor, or
press F6 to open the row properties dialog, enter the desired text in field "Text", and click "Ok", or
enter the desired text in field "Text" in the properties view in yEd's lower right corner.