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Argh. Answering my own question here.
Looks like ⌘ + = on Mac is a keyboard shortcut that does this. I must have inadvertently triggered this and hit the wrong button on the subsequent dialog because I didn't understand what had happened. Fortunately no significant data loss.
Related, would be nice if there is a way to deactivate unused keyboard shortcuts.
If you create a custom menu (you need FM Advanced) and remove the menu item its shortcut will also be removed.
Yes, what you experienced is the expected outcome of using Replace Field Contents. Beware of this command; it is a sledge hammer which can cause chaos if used without understanding. It has its place in the FM toolbox, but you should study it so you know what that place might be. See: