This looks like a PDF page-rotation mismatch: the second page is likely stored in landscape with a rotation flag.
PDF24’s watermark tool applies the watermark to the raw page coordinates, so it appears rotated.
Fixes / work-arounds:
Pre-normalize the PDF pages (e.g. with pdftk or qpdf --rotate=0:all in.pdf out.pdf).
Or use another watermark tool that respects page rotation.
Report the issue (include the sample file) so PDF24 can adjust watermark placement for rotated pages.