Originally Posted by Blossom
Do this Convert the Sigil epub to a Calibre epub then try to merge. My guess is Sigil could be messing up the navigation file. Running it through Calibre first before merging should fix it.
I honestly think that simply using a Calibre plugboard to number your books in series order is a much easier way to look after a series. No need to use a collection for the purpose at all then.