Doug Hickling suggests:
" I want to suggest that you revise your listing of John (Sir) (of
Ardgowan) Stewart, whose daughter Margaret married Duncan (Sir) Campbell and became the mother of Colin (Sir) (1st of Glenurchy) Campbell. You correctly show that Sir John's father was Robert III (John) Stewart, King of Scotland, but the authorities say that John (Sir) was the natural child of the king and his mother's identity is unknown.
THE SCOTS PEERAGE volume 1, at 331, refers to John (Sir) Stewart of Ardgowan as a "natural son of King Robert III."
Walter Lee Sheppard, Jr., in his article ROYAL BYE-BLOWS III--ILLEGITIMATE DESCENDANTS OF THE SCOTTISH KINGS TO JAMES VI, NEHGR, October 1968, at 265, lists both James Stewart of Kilbride and John (Sir) Stewart of Auchingowan and Ardgowan as "known bastards" of King Robert III.
Alastair Campbell of Airds in his A HISTORY OF CLAN CAMPBELL, VOLUME I, at 121, says that John (Sir) was a "natural son" of King Robert III. Brian Tompsett agrees. He shows only a father for John (Sir) and does not include him in the list of children of King Robert III and his spouse Annabella Drummond. Considering that the fact of his illegitimacy is so widely recognized, I think it really strange that so
few gedcoms on RootsWeb have picked it up."