If I append several files one after the other with

pdfcpu merge -mode append a.pdf b.pdf
pdfcpu merge -mode append a.pdf c.pdf
pdfcpu merge -mode append a.pdf d.pdf

the bookmark list becomes more and more nested:

-a
--a
---a
---b
--c
-d

What I expected:

-a
-b
-c
-d

Can this be fixed by not creating a new bookmark for the original document every time?
Thanks for your help.