Shapely: parallel_offset and buffer don't close contours correctly

Expected behavior and actual behavior.

Sometimes, when using parallel_offset or buffer, the resulting contour is not closed correctly. The point where the contour is not closed correctly always seems to be the first/last point of the contour.

The following is an issue I initially posted on StackExchange.

Steps to reproduce the problem.

Here is a minimal working example that produces a contour which is not closed when using parallel_offset. When using buffer, a rounded edge is created (despite the mitred style).
Curiously, in my larger dataset there are other very similar contours which close successfully.

import matplotlib.pyplot as plt
from shapely.geometry.polygon import LinearRing

def plot_line(ax, ob, color):
    x, y = ob.xy
    ax.plot(x, y, color=color, alpha=0.7, linewidth=3, 
            solid_capstyle='round', zorder=2)

polygon = [[-29.675, -30.675],
           [-28.4094, -29.4094],
           [-28.325, -29.325],
           [-28.325, -29.764],
           [-28.325, -29.7933],
           [-28.4587, -29.8274],
           [-28.4676, -29.8297],
           [-28.5956, -29.8814],
           [-28.6041, -29.8848],
           [-28.724, -29.953],
           [-28.732, -29.9576],
           [-28.8417, -30.0413],
           [-28.849, -30.0469],
           [-28.9466, -30.1445],
           [-28.9531, -30.151],
           [-29.0368, -30.2607],
           [-29.0424, -30.268],
           [-29.1106, -30.3879],
           [-29.1152, -30.3959],
           [-29.1669, -30.5239],
           [-29.1703, -30.5324],
           [-29.2044, -30.6661],
           [-29.2067, -30.675],
           [-29.6457, -30.675],
           [-29.675, -30.675]]

poly_line = LinearRing(polygon)
poly_line_offset = poly_line.parallel_offset(0.05, side="left", resolution=16, 
                                             join_style=2, mitre_limit=1)
# Alternative:
# poly_line_offset = poly_line.buffer(0.05, resolution=16, join_style=2, mitre_limit=1).exterior

fig = plt.figure()
ax = fig.add_subplot(111)
plot_line(ax, poly_line, "blue")
plot_line(ax, poly_line_offset, "green")
plt.show()

Operating system

Windows 7, Python 3.6.2

Shapely Version and Provenance

Shapely 1.6.3 (installed from PyPI)