after investigation our code and sample document, I see that reason of the issue is in concatenation matrix operator in the start of the page contents:
1 0 0 -1 0 841.889764 cm
i.e. all page contents is rotated (including image you added).
You could add GSave operator (q) in the start of the page contents and GRestore after its end, something like that:
page.Contents.Insert(1, new Aspose.Pdf.Operators.GSave());
or update your matrix according yo the page contents rotation.
I’ve created ticket PDFNET-53568 for more detailed investigation of the issue.