Glyph rendering voodoo

Well, probably not voodoo but very strange to me. Try this:

  echo "\\\f[U-S]\[uF8E8]\f" | groff -Tpdf | pdftotext - -
  
And if you see the same as me, you'll see a white circle with an
intersection symbol in it.

Now, try instead and send the same output to a file:

  echo "\\\f[U-S]\[uF8E8]\f" | groff -Tpdf >t.pdf
  
Open the file in your preferred pdf-viewer and you should see a circle with
an 'R' in it (registered trademark?), i.e. a completely different glyph.

pdftotext should not be responsible because groff has already done its job.

If you see the same, could you explain this to me - or maybe just slap my
face so it points in the right direction ;)

Regards,
Morten