Concatenating PDFs using PDFtk leaves all bookmarks open

Question

I am using pdftk to combine multiple PDF files into a new PDF using a cmdline like

pdftk al.pdf dz.pdf as.pdf ao.pdf ai.pdf ag.pdf ar.pdf aw.pdf au.pdf az.pdf cat output all-a.pdf

All the input PDFs have all their bookmarks closed. But in the newly created output pdf all the bookmarks are open.

Is it possible to maintain the state of the bookmarks somehow?

Thanks