The Undo function erases more than the last action. It may erase an entire reaction, for example.
We have identified a bug in ChemDraw 19.1 when using the Ctrl-Z or Undo function on Windows.
Until the bug is fixed, a possible workaround is to close and reopen the ChemDraw document.
If the issue persists, please contact Informatics Support, otherwise we will post an alert on the News section of the Support site when a new release is available.