I’ve searched this online but I don’t think I’m wording it correctly. Thank you in advance for this it’s driving me nuts!
Maybe because you are pasting from an html source?
Or what do you mean “changes”? Changes from what to what? Can you post a screenshot?
If you want to paste without formatting, ctrl-shift-V should be the shortcut on Windows. Cmd-shift-V on MacOS I think.
To add to rigamarole’s answer: You can also right-click “paste as plain text”. Also, check the indentation of things you are pasting. If they are too indented compared to the content around them, I think Obsidian will interpret them as code, equivalent to putting text inside ` which it changes color.
Does any of this help?