Some more info. Having seen Doc's post, I've just looked at my Series groupings.
Step1. Of 106 books, 2 appeared under the wrong Series first letter. Both had Series value Inspector Alan Grant, but appeared under 'G' instead of 'I'. This is very odd as I also have books with Series values Inspector Banks and Inspector Vaara which are both correctly grouped under 'I'. In the calibre library Tag Browser panel, all these books are correctly grouped under Series 'I'.
My Tweaks, from day 1 of them being available, have been set as:
title_series_sorting = 'strictly_alphabetic'
save_template_title_series_sorting = 'strictly_alphabetic'
and have never changed since.
Step 2. In calibre I opened Edit Metadata and saved (with no changes) so that the metadata.opf file has today's date. After reconnecting CC, both books are still incorrectly grouped under 'G'.
Step 3. In calibre I changed one of the two books to have Series value Inspector Grant. After reconnecting CC this changed book is now correctly grouped under 'I'. Then I changed the 2nd book in the same way. Both books now correctly grouped in CC.
Step 4. In calibre change both books back to their original Series value of Inspector Alan Grant. Reconnect CC, both books still correctly grouped under 'I' with their original value of Inspector Alan Grant.
Conclusion, I've no idea
Edit: I've now looked through all my other first letter groups (Title, Authors, Tags, pubdate, #awards) and have not found any similar problems.