Added a custom stamp - it's sideways - how to rotate?

You might need to edit the stamp design in your software. If you're using a graphic tool, try rotating the text before saving it. If it's in a PDF editor or a stamping tool, check for an option to adjust the orientation. Let me know which software you're using, and I can suggest more specific steps!

It's absolute nonsense and very bad programming. It feels like this has been done deliberately to upset users and make life difficult and waste time. This is expensive software that doesn't work properly and I am appalled.

Has this been solved yet? I have had the same issue for some time now. The image is right-side up in AutoCAD and remains so when published as a .pdf, then ran through Illustrator and then publishing as a .png from AI (this is to illustrate that the .png remains right-side-up, although only the .pdf publish is used to create the Adobe Stamp). It remains right-side-up as as a thumbnail in Adobe Acrobat DC when browsing to the .pdf in the "Select Image for Custom Stamp" dialog box, and still under the "Create Custom Stamp" dialog box after import. The first instance of a 90° rotation occurs when actually selecting the stamp for placement on the page, or in the thumbnails of created stamps through selecting "Custom Stamps / Manage". The .pdf imports into Bluebeam Revu right-side-up as well (though the image does note a rotation of 270° in that software).

Just for a test, I did rotate and save the .pdf page in other orientations. The rotation is reflected in the .pdf; however, the stamp always reverts back to the same sideways (left readable) orientation when selecting the stamp to place. Additionally, I republished the original .pdf from AutoCAD using a landscape orientation, rather than portrait (the plot settings do include an icon that rotates 90° when switching between the two; however, the plot preview and produced .pdf both have a right-side-up orientation for either page orientation setting).

I of course can, manually rotate the image upon placement, but the default placement is what I would like to address.

You can change the stamp by rotating the page in the stamp file. I don't mean rotate the view. The actual page needs to be rotated

To rotate a stamp click it and then click the little circle that appears at the center of the top border, and drag to rotate the stamp. Hold down Shift to make it rotate in intervals of 45 degrees.

So you are saying that the stamp was created in the correct "horizontal" orientation, but when it is placed it is rotated 90degrees? If so this is because the PDF page is rotated.  Stamps are placed according to the page rotation. Try placing it on other pdfs that are not rotated.