import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle
import numpy as np

xr = np.linspace(-10, 10, 2)
X, Y = np.meshgrid(xr, xr)

# ncols needs to be >= 3 to trigger the problem
fig, axes = plt.subplots(nrows=1, ncols=3, sharey=True)

for iax, ax in enumerate(axes.flatten()):
    ax.set_aspect('equal')
    r = Rectangle((X.min(), Y.min()), X.ptp(), Y.ptp(),
                  color='k', fill=False, ls='--', lw=1)
    ax.add_patch(r)
    ax.set_xlim(X.min()-1, X.max()+1)
    ax.set_ylim(Y.min()-1, Y.max()+1)
plt.savefig('export.pdf')
plt.show()