I've done a little more testing on the problem of LO not opening the documents in which I'd selected text and added comments to it.
It appears to only affect the LO User Guides documents. Perhaps it is something to do with their origins, or the template. I've no way of knowing.
However, I can create a new document from the default template and the new feature works perfectly properly.
Using the feature in a User Guide causes LO to return an error when attempting to open the saved document.
JohnS