I can reproduce this bug, and it isn't present on 1.5.3

This is likely something fishy with mathtext...

Bisects to 6bfd8b1

This isn't just isolated to tick labels. It's anywhere you put in math expressions in a byte string. Add plt.title(r'byte string with $m_at_h$') to the example code above and it will show that the text switches from serif to sans-serif for the rendered math.

what do you mean by 'byte string'?

In my case this can be solved with:

from matplotlib import rc
rc('mathtext', default='regular') 

which will use your specified font for math text, such as the $10^{x}$ being used for logscale tick labels.

I think this has to do with the mathdefault shim for for preserving the classic style. Using just 2.0 defaults the LogFormaterMathtext ignores font.family. After style.use('classic') they are respected.

Actually I think the shim is wrong and uncessary:

def _mathdefault(s):
    """
    For backward compatibility, in classic mode we display
    sub/superscripted text in a mathdefault block.  As of 2.0, the
    math font already matches the default font, so we don't need to do
    that anymore.
    """
    if rcParams['_internal.classic_mode']:
        return '\\mathdefault{%s}' % s
    else:
        return '{%s}' % s

The assumption of that the math font equals the regular font is not true in general.

As a workaround

import matplotlib.ticker
matplotlib.ticker._mathdefault = lambda x: '\\mathdefault{%s}'%x

Removing this shim and always using '\\mathdefault' seems like a reasonable change.

Closed by #8354

I've noticed the workaround suggested doesn't work for cases where tick labels like "2 x 10³" appear. These are styled with dejavu sans and there doesn't seem to be anything I can do to change them. They don't seem to use the usual mathtext or even roman

import matplotlib.ticker
from matplotlib import rc
matplotlib.ticker._mathdefault = lambda x: '\\mathdefault{%s}'%x
rc('mathtext', fontset='custom')
rc('mathtext', default='regular')
rc('mathtext', rm='serif')
font_path = 'C:\Windows\Fonts\Roboto-Bold_0.ttf'
font_prop = font_manager.FontProperties(fname=font_path, size=11)
...
for label in (ax.get_xticklabels() + ax.get_yticklabels()):
    label.set_fontproperties(font_prop)
plt.xlabel(r"Regular txt, ${math txt, \mathrm{rm math txt}}$", fontproperties=font_prop)

@binaryfunt Can you open a new issue for that?