The copied content is there, but there is a bug in klipper that causes the clipboard content to remain hidden from view upon the first click on the icon. You have to open the clipboard twice in order to see the contents.
This bug has been around since we switched to Plasma 5.27, but it may have already been introduced in 5.26 — I can’t say for sure because I stayed with 5.24 until we switched to 5.27.