If I'm entering CMD-Z in a field with content, like caption, keywords etc, and there is nothing to undo, the field is cleared.
How to reproduce:
1: Open a IPTC dialog, either 'Metadata (IPTC) Info for ...' or 'Metadata (IPTC) Template'
2: Place curser in any field with content
3: Press CMD-Z
4: Watch the content get zapped...
Also, if the field have been changed, continuesly pressing CMD-Z will finally blank the field.
PM Plus Build 6474
macOS 10.15.7
/Michael