While upgrading Cartopy's test images (SciTools/cartopy#354), it seems that the zorder of the lines in stream plots has changed. Originally, the lines used the default zorder from LineCollection, but as a result of #2225, the lines use the zorder default set for the method. The default zorder from LineCollection is 2; the default for the method's argument is 1.

If you look carefully at the plot, you can see that the coastlines move from below to above the stream lines:

Z order is tricky, being a float, so I'm not whether adjusting the default won't cause issues.