Same behaviour for me also.
Just updated this morning to Collabora Online - Built-in CODE Server Version 25.4.504 and the copy/paste works from within the document but doesn’t work from outside
When using ctrl+v, the page flashes but nothing pasted. If you let the keys ctrl+v down for some time then it eventually works but paste several occurrence of the text
I have the same issue. Pasting from within the document works fine, but not from the system. I can paste perfectly well into other applications such as Nextcloud notes, but not Collabora with CODE.