To reproduce:
-add grid pane to the content panel
-add button to cell (0,0) (first col and first row)
-select the button and increase row span up to 3 by typing 3 in the inspector's panel
-select grid
-select and remove last row using button "-" at layout panel
-select button

The row span has been reduced to 2 (you can see that id you save) but the inspector still shows 3