When trying to add a gradient node on canvas by double-clicking, and there is an overlaid object with some gradient, the topmost object gets edited instead of the node addition on the selected object.
See the following demo:
The bottom circular object is selected, having a radial gradient. Cursor shows a + sign showing that double clicking on the gradient will add a node, yet linear fill gradient is set on the checkmark object above.
This behavior is there sometime since the introduction of the awesome canvas editing.