This isn’t a bug, but it’s something I noticed and can’t find mentioned in the manual. If you have text in the document selected, the Auto-Generate Synopsis button in the Inspector will use that selected text for the synopsis, but the same command via the Documents menu will use the beginning text of the document just as though no text were selected. Since the menu command is available for multiple documents, that may mean it can’t use selected text, but since the manual calls the two commands “synonymous” (§18.1), it might be worthwhile to mention this difference. Or changing it so the menu command uses selected text, if that’s possible.
Yes, that’s a mistake in the manual (Ioa… ). The actions aren’t synonymous - the one in the Documents menu can be applied to multiple documents and so ignores selection; the one in the inspector bar only affects the current document and so will use the selection if there is one.