[Python] Why is there grid lines for some plots and not for others?

Consider this MWE,

from matplotlib import pyplot
pyplot.style.use('classic')
pyplot.rcParams.update( {
'text.usetex': True,
"font.family": "serif",
'text.latex.preamble': r'\usepackage{amsmath, amssymb, mlmodern}', } )
import numpy


fig, ax = pyplot.subplots(3,3)
fig.tight_layout()
fig.subplots_adjust(hspace=0.1, wspace=0.1,
left=0.09, right=.95, top=.95, bottom=.09)

x = numpy.linspace(0, 8, 100)

for i in range(3):
for j in range(3):
ax[i,j].plot(x, numpy.sin((1+j)*x+numpy.pi*i), )
ax[i,j].grid(which='both') # <----------------- I added grid here
if i!=2: ax[i,j].set_xticks([])
if j==1: ax[i,j].set_yticks([])
if j==2: ax[i,j].yaxis.tick_right()

ax[0,0].set_ylabel('$\phi=0$')
ax[1,0].set_ylabel('$\phi=\pi$')
ax[2,0].set_ylabel('$\phi=2\pi$')

ax[2,0].set_xlabel('$f = 1$')
ax[2,1].set_xlabel('$f = 2$')
ax[2,2].set_xlabel('$f = 3$')

pyplot.savefig('waves.png')


Which produces the following plot,

I don't understand why matplotlib has grid lines for (0,0) and (0,2) axes, vertical grid lines for axes (0,1), horizontal grid lines for axes (1,0), (1,2), (2,0) and (2,2) and no grid lines for axes (1,1) and (2,1).

How do I make sure that all axes have the grid similar to (0,0)? Thank you.

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