Thanks for your quick response.

But I think the title might not describe the issue very well. The current math_fontfamily implementation is irrelevant to rcParams['mathtext.fontset']. A better title might be Error while setting font without setting math_fontfamily

I think the problem is: If you set font manually, you have to set math_fontfamily manually as well.

plt.text(1, 7, msg, size=12, font='Arial')  # math_fontfamily='cm'

It works fine if you set math_fontfamily, which is my current workaround.

plt.text(1, 7, msg, size=12, font='Arial', math_fontfamily='cm')  # good

My expectation is if I don't set math_fontfamily='cm', it should use matplotlib.rcParams['mathtext.fontset'] as default.

I was able to determined that the bug stems from the changes done here #18862

If I change to the previews version, the code will work. Unfortunately the PR in question is not very descriptive on the reasons behind this change. Perhaps someone involved at the time can help me? @anntzer

https://github.com/matplotlib/matplotlib/pull/18862/files#diff-aca801331b4c856bff720af0575b9ad63f402c893abd7d939e5547ba961ee324L900-L902

The removed 2 lines (900 and 901) might be the reason. In my debugging, the fontproperties in the Text instance is Arial with _math_fontfamily=None (while it should not be None).

@ain-soph I edited the title to make it more descriptive. I can change it for a better one if you have suggestions.

So the previous solutions of 3.3 is lazy, which means to save self._math_fontfamily=None at set_math_fontfamily, and change to rcParams['mathtext.fontset'] at get_math_fontfamily.
The author change the behavior to creation time, set self._math_fontfamily=rcParams['mathtext.fontset'] at set_math_fontfamily.
But it seems not always working, sometimes after calling set_math_fontfamily at initialization, self._math_fontfamily is set back to None by some other operations. So it throws exception when check_getitem.

I try to add some debug print

Here's the log before exception:

......
2251559120368 set cm
2251559120560 set cm
2251561657872 set cm
2251561659120 set cm
......
2251563080192 get cm
2251563155856 get cm
2251563157728 get cm
2251563159504 get cm
2251559120560 get None
2251559120560 get None
2251559120560 get None
2251559120560 get None
<Exception>

We can see the error occurs at id 2251559120560 because the _math_fontfamily is still None. But If you search 2251559120560 you'll find its self._math_fontfamily is set to cm previously (at very beginning). So there's some operation that modifies self._math_fontfamily to None in the middle.
Not quite understand why it works well when you pass the argument math_fontfamily='cm'. Hope this information helps.

    @property
    def _math_fontfamily(self):
        print(id(self), str(self.__math_fontfamily))
        return self.__math_fontfamily

    @_math_fontfamily.setter
    def _math_fontfamily(self, value):
        previous = str(self.__math_fontfamily)
        self.__math_fontfamily = value
        after = str(self.__math_fontfamily)
        print(id(self), f'{previous:10} {after:10}')

I make it a property so that I can get noticed once self._math_fontfamily is being called or set.
However, I can still only see the problematic fontproperties setter called only once with correct value cm.
When it get called by get_math_fontfamily, it is None at that time.
Mysterious, I don't know when it changes value. And how's that possible to change value without called setter function.

Log:

......
1816804568992 None       cm
1816804568992 cm
1816804568992 set cm
......
1816804568992 None
1816804568992 get None
1816804568992 None
1816804568992 None
1816804568992 get None
1816804568992 None
1816804568992 None
1816804568992 get None
1816804568992 None
1816804568992 None
1816804568992 get None
1816804568992 None

Have found the issue, will submit a PR (very small fix) soon.

May close this issue. @diegopetrola

Congratulations on solving your first issue :)