A clipboard content has a display format. The developer who designed the software is supposed to provide that display format. Without it, you cannot view the clipboard content from the clipboard viewer.
Windows clipboard viewer supports 4 display format. For your case, the app doesn't use any of the 4 display format, and the developer didn't provide a display format. As you mentioned, it is undocumented, maybe it never worked.
If you are technical and want to explore the data, the clipboard data is OLE object, you may lookup the COM registry to find out more clue. It is one of the Class IDs in your Windows Registry. Usually starts with the application name, again, it all depends on the developer to write in good faith.
The four display formats are: CF_DSPBITMAP, CF_DSPMETAFILEPICT, CF_DSPTEXT, and CF_DSPENHMETAFILE. These display formats are rendered in the same way as the standard formats, which are: CF_BITMAP, CF_TEXT, CF_METAFILEPICT, and CF_ENHMETAFILE.
https://docs.microsoft.com/en-us/windows/win32/dataxchg/clipboard