@ceciliatan Only thing I can think of is that the default encoding for TextEdit changed for some reason β maybe an OS update?
So what I was pointing to was the second option on the page which lets you set the default encoding for TextEdit. I was hoping that playing around with it might help you find an encoding where any pasted in text just works.
But on the other hand, it might not be the encoding at all. Unfortunately, it's a little difficult to say without being able to try things out in person ... at least for me π
The other option would be to switch to a plain text editor. Theoretically, when you paste into that, all text should show up as normal but that might not suit your needs.