Joanna Isaac - Who is John Stewart of Dalswinton (1315-1380) and who did he marry?

Another riddle and once more, any help would be appreciated.

On this and a couple of other sites, John Stewart of Dalswinton (1315-1380) married Joanna Isaac.

He was the grandson of Sir John Stewart of Bonkyl, the son of Sir Walter Stewart of Garlies & Dalswinton.

She was the granddaughter of Robert the Bruce and Elizabeth de Burgh, the daughter of Thomas Isaac and the princess of Scotland, Matilda/Maud Bruce.

Yet despite three days of researching almost full time, I can find no historical record that confirms this marriage and given the royal connections, I wish to be very, very cautious.

Can you point me towards any such record? How have you arrived at the conclusion that John Stewart of Dalswinton (1315-1380) married Joanna Isaac?

Thank you.