I have further isolated the bug to one specific Pages document.
The document is just over 30,000 words with a couple of dozen images and is 15.9MB. I duplicated the file, and the fault still occurred. I deleted the images. No change. I deleted over half the text. No change. It wasn't until the word-count dropped below about 13,400 that the fault disappeared and the spell checker suggestions showed up again in the right-click menu. I could reproduce the bug by cutting and pasting a few paragraphs to take the word-count over 14,000 but the error didn't trigger at a consistent number (or word).
I have tried creating a new file and filling it with over 14,000 words by copying and pasting a chunk of new text multiple times, and the fault hasn't reproduced.
I have opened a new, blank file and copied and pasted from the original file into it and the bug HAS returned - and again, disappears when I reduce the word-count down below around 13,400. So there's something in that file that is somehow affecting the dictionary look-up when the word-count goes over an arbitrary limit. I've moved on to a new file and that one will be archived and so the bug isn't important to me at the moment.
If the bug recurs with the new file, I'll post again here.