The background is realized through an internal FancyBboxPatch (Text._bbox_patch). It is initally None and only created if you want a background or border around the text.

What happens here is that the patch is created with finite linewidth 1 and the edgecolor is the same as the facecolor (One can debate whether that behavior is reasonable, but it likely needs a closer look to handle all potential cases reasonably). To completely undo txt.set(backgroundcolor='red') you should remove the background patch via txt.set(bbox=None) instead of making the patch transparent.

Note: The behavior is due to

There is a need to do something about the edge because by default rcParams["patch.linewidth"] is 1. The alternative to setting edgecolor to the same color as facecolor would be to set linewidth=0. That might be the slightly better approach. OTOH it will lead to subsequent txt.set_bbox(edgecolor=...) have on visual effect. So that's not ideal either. Overall, I believe it's not worth changing the behavior and the improved documentation in #29875 should suffice.

Thanks for your reply.

For an user of matplotlib like me it is hard to guess, that the problem of the red border results from the initialization of the text object and how to work around.
At least you should point out in the manual, that "txt.set(backgroundcolor='none')" as a command before is a simple workaround. Or something like this.

I got another problem, probably caused by the initialization of text object, which is more hard to understand for the user.
It appears when you try to get the width of the text. Sometime it works, sometimes there is an exception:

----------------------------

import matplotlib.pyplot as plt

fig, ax = plt.subplots(1, 1)
fig.set_dpi(100)

txt = ax.text(0.1, 0.8, 'My Text ...')
try:
width = txt.get_bbox_patch().get_width() # You will get an AttributeError:
except AttributeError:
print("'NoneType' object has no attribute 'get_width'")

Make it work with the "workaround of backgroundcolor":

txt.set(backgroundcolor='none')
width = txt.get_bbox_patch().get_width() # Now you will get the width

plt.show()

There is a need to do something about the edge because by default rcParams["patch.linewidth"] is 1. The alternative to setting edgecolor to the same color as facecolor would be to set linewidth=0. That might be the slightly better approach. OTOH it will lead to subsequent txt.set_bbox(edgecolor=...) have on visual effect. So that's not ideal either. Overall, I believe it's not worth changing the behavior and the improved documentation in #29875 should suffice.

Could we instead set edgecolor to "none"? If I understand correctly, that is the default for a patch these days anyway (though it is black in classic mode).

Thanks, that's a good idea and probably the correct solution.

The only downside is that the size of the box changes (left half is edgecolor none):

This is a breaking change. It's probably still worth doing so and communicating it as a fix.

Another option might be to set the edgecolor to match facecolor here

Then set_backgroundcolor would be the same as set_color directly on the patch

That would be a bit counter-intuitive to me. My mental model is that the text background color is only the fill and does not affect the frame. E.g. when I have

text.set_bbox(dict(facecolor=('red'), edgecolor='black'))
text.set_backgroundcolor('green')

I would expect that the black frame stays.

I also like that the box has sharp color edges when no edge line is used. The boundary line is antialiased and blurs the edges.