I hate to speculate. It places bad research on the web which helps none of us.
But I do believe that Joanna Isaac might well have been referenced as Jonet and even Joan maybe?
Eugene Thomas states that Joanna Isaac was daughter to Thomas and Maud Bruce. Given the royal connection, we should be very certain of that if we can. What do other sites say on that?
That takes me to who Janna married. I see some sites say her husband was John MacDougal, Lord of Lorne. Others imply that Joanna married 2x her other husband being John Stewart of Dalswinton.
With Joanna and John Stewart possibly mother of Katherine who married Herbert, Lord Maxwell.
I would really like to test this. Seems that Eugene is more tach savy than I am so any help will be very much appreciated.