Seems like you cannot set the linestyle for arrows via tuples. This just works fine

plt.annotate("",
xy=(0, 0), xycoords='data', xytext=(1, 1), textcoords='data',
arrowprops=dict(arrowstyle="->", connectionstyle="arc3", color='0.5', ls='--')
)

where the bold part sets the linestyle of the arrow. However, this won't work and throws an error when using the tuple description of linestyle:

plt.annotate("",
xy=(0, 0), xycoords='data', xytext=(1, 1), textcoords='data',
arrowprops=dict(arrowstyle="->", connectionstyle="arc3", color='0.5', ls=(0, [2 ,2]))
)

The error says

TypeError: unhashable type: 'list'

The same is true for plt.arrow. The tuple description works just fine on regular plots:

xx = np.linspace(0, 1, 100)
plt.plot(xx, xx, ls=(0, [2, 2]))

Am I doing something wrong or is this an actual bug?