Yes, I have noticed the same, and the “@” is the cause. Discourse tries to find the user you are referencing with @, which disrupts the formatting. I can switch off this behavior for the entire forum, which I don’t want to do, so the only alternative, as you have found out, is to add the text as a code block.
I can paste into code blocks without problems. The important thing is to create a code block first and then paste into it, not to paste the text and then make it a code block. How did you proceed when you got the problem in code blocks?