I have a simple PDF with a text line and a Form XObject containing an image (in his own XObject of type Image). Accessibility checker says "Other elements alternate text failed". But looking at the file via Notepad and Irups the file is well formed as per specification and also PAC3 validates it fine. By the way, if I substitute the image in the XObject with a text, the file is correctly validated.
Is this a known limitation of Adobe accessibility checker?