README.txt silently started showing red error blocks

[Nakilon]

I have a repo that is almost two years old and its README did not need fancy formatting so I made it .txt

But I accidentally discovered that since some day it does not render correctly anymore. Instead of being a plain text it's now trying to show some "math macros". Here is an example of a broken fragment:

And how it's now rendered on the main page of the repo:

I've added

$ cat .gitattributes 
README.txt linguist-language=Text

but it did nothing.