Longitude labels not showing correctly with polar projection

Description

The labels of longitude show correctly in v0.19.0, but do not work well in v0.20.1.

Code to reproduce

import numpy as np
import cartopy.crs as ccrs
import matplotlib.pyplot as plt
import matplotlib.ticker as mticker

ax = plt.subplot(111, projection=ccrs.NorthPolarStereo())
gl = ax.gridlines(color='r', draw_labels=True, x_inline =False, y_inline =True)

polar_extent = [-180, 180, 60, 80]
ax.set_extent(polar_extent, crs=ccrs.PlateCarree())

gl.ylocator = mticker.FixedLocator(np.arange(60, 90, 5))
gl.xlocator = mticker.FixedLocator(np.arange(-180, 180, 60))

Traceback

Version 0.19.0

Version 0.20.1