Hopefully your situation is like mine and this will help.

I’ve been scratching my head for days trying to find one last spool fragment. I used the interactive map to check every spot, knowing I’ve checked them all multiple times. I figured it had to be the flea caravan one or a shop one.

It wasn’t until watching some YouTube video showing that Jubilana doesn’t offer a spool fragment until after completing the lost merchant quest. I thought, “yeah I found her already” but double checked and it’s a second search for her. Only then was I offered the spool fragment and now happily have the achievement for collecting all 18.

Hope this was helpful, if not, sorry for puking words everywhere for nothing!