When copying and pasting text that goes inside code blocks they should be pasted as plaintext by default, I can’t think of a reason where converting to markup inside a code block is the desired outcome or even if it is even common. That might be my bias flying high, so correct me if that is a wrong assumption.
I know we have context menu and shortcuts to paste as text, but doing the right thing™ automatically is a better experience. If the cursor is inside a code block pasting should automatically paste as plaintext.
Is this the end of the world? No, but if my assumption is correct - that inside code block we always want plaintext - it is just much nicer to use.
Current workaround (optional)
Use the shortcut or context menu after you’ve already pasted and Obsidian did the wrong thing by inserting a bunch of markups inside the code block.