The output preview tool is a seemingly simple, yet in practice a complicated and unintuitive beast (however that is another topic)!
If the PDF is a PDF/X variant, then it will have an “output intent” - the dropdown menu will have the key words “output intent” listed before the profile name. This is by design and should be a good thing. The output intent profile is also named and listed above all other profiles in the drop down menu.
For “plain” PDF files that don’t have an output intent, the working space of colour settings will be listed here in the drop down menu. On all the systems that I have used, this will always default to the CMYK workings space set in Acrobat, even for files that only contain RGB (say a MS Word to PDF output).
Stephen Marsh