I'm away from home so I can't check specifics, but it looks to me like this is simply a matter of an army being able to hold that many divisions w/brigades plus a full corps. Somewhere in the manual it says what the limits are. It doesn't look like a bug, though I might be missing something.
Try taking one or two divisions out of the army -- the corps should go in nicely, and it might even accept more brigades.
You may very well be right. Although I think I moved brigades attached directly to a Corps out of the Corps, and still couldn't max out a Division in that same Corps to 6 Bdes.
Have to try this tonight some more, and try and find the limit of Brigades within a Corps. I think it might be 12 or 18, but am not sure.
'What is more amazing, is that amongst all those approaching enemies there is not one named Gisgo.' Hannibal Barcid (or Barca) to Gisgo, a Greek staff officer, Cannae.
That's the CSS North Carolina BB-55
Boris Badanov, looking for Natasha Goodenov