Name pretty much tells it, I was scrolling through the GTS and
found these two bugs:
First one we can see sprite disappearance, on second, the error,
that causes shadow Pokemon to appear normal in offer window.
Unfortunately, I can't put second one as an image here, so
basically we have only one portion of the bug.
The reason for some missing sprites is because Shadow Unown's do
not have sprites yet so they show up as a broken/missing image. (A
few other pokemon also have missing or broken images and Riako is
aware of them.)
As for shadow pokemon appearing normal once your click on a trade,
its been happening since shadows came out, I do think it would be
nice to see that they are shadows when on the offer screen and
including the shadow star on them but its something that either
hasnt been implemented yet or something that needs fixing.
The bug with missing Unown shadow sprites has been fixed in the
past, but the other bug (shadows appearing as normal 'mons) still
needs to be fixed and I've put it on my ToDo stack.