Hello!

I need more info about this.
Do you also get rotated images when you just pdfcpu import them?
Maybe you can post a screenshot.

With import it is not rotated.

screenshot

Can you send me both result pdfs (from grid and import) for analysis?
Otherwise I have to guess what I am looking at.

aaa.pdf
abc.pdf

Have you tried switching the grid dimensions?

pdfcpu grid -- "bo:off" out.pdf 1 3 l1.png l2.png l3.png
instead of
pdfcpu grid -- "bo:off" out.pdf 3 1 l1.png l2.png l3.png

Also:
aaa.pdf is the result of ?
abc.pdf is the result of ?

Using
pdfcpu grid -- "bo:off" out.pdf 1 3 l1.png l2.png l3.png

13.pdf

aaa.pdf is result of
pdfcpu import aaa.pdf aaa.png

abc.pdf is the result of
pdfcpu grid -- "bo:off" abc.pdf 3 1 l1.png l2.png l3.png

nup and grid are enforcing a best-fit orientation of individual content onto a page when rendering.

If you get the latest commit, then you can use enforce:off to achieve what you want which is keep the original orientation of the content.