Steps to reproduce:
- Open a page for editing in NWE.
- Copy from any article some HTML with a link.
- Paste in NWE. As expected the content is transformed into wiki syntax.
- Press Ctrl-Z.
Expected: The pasted content is removed again.
Actual: Nothing happens.
Undoing via toolbar works, undoing via keyboard for pastes without conversion works, too.
I tested in Firefox, both de.wikipedia.org and de.wikipedia.beta.wmflabs.org.