Is the line style supposed to depend on the export backend?

MPL 1.5.1 is from Anaconda and MPL 2.x is from Github, both on Linux (CentOS 7)

With MPL 1.5.1, I get the same line styles between both PDF and PNG exports:

• PNG @ 100 DPI:

• PDF @ None DPI: MPL_ver_1.5.1_None.pdf

while with master version, they are different between PDF and PNG exports:

• PNG @ 100 DPI:

• PDF @ None DPI: MPL_ver_2.0.0b1.post1628+g1aba7c3_None.pdf

NB: the problem seems to be independent from the DPI value: I get similar line styles between both PNG exports with 100 DPI and 300 DPI.

• PNG @ 300 DPI:

The example snippet is

import matplotlib
import matplotlib.pyplot as plt
import numpy as np

test_case = matplotlib.__version__

fig, ax = plt.subplots(num=test_case, figsize=(4.8, 3.2))
ax.set_title("MPL version: {0:s}".format(test_case))

xx = np.array([0.0, 1.0])

ax.plot(xx, 1 * xx, ls='dashed', label='default lw')
ax.plot(xx, 2 * xx, ls='dashed', lw=2, label='lw=2')
ax.plot(xx, 3 * xx, ls='dashed', lw=3, label='lw=3')

ax.legend(loc='upper left')

plt.show()

for ext, dpi in (('pdf', None), ('png', 100), ('png', 300)):
    fig.savefig("MPL_ver_{v:s}_{d}.{e:s}".format(v=test_case, d=dpi, e=ext),
                dpi=dpi)

My guess would that I may be somehow related to the new behavior introduced in #5926, but I didn't have time to have a look yet.