After duplicating entries to a second 1password account, many of the attached files are disappearing

@vintageroses

This sounds like the items are not attached but more related items. If that's true you'll need to move the related item over too and then relink the two together. I think of related items as shortcuts saved in an item that point to the original. They will get broken when moving between account for example.

Attached items/documents etc. reside entirely within the item. They will move entirely with the item even if moved between accounts.

Do you see the name related item in your originals?