Bartrum originally ascribed Tangwystl as Gwladys’s mother, but then took that connection out, which is why we connect her to “unknown mistress,” of whom Llewelyn had several. On the page of his charts we have here — https://www.geni.com/documents/view?doc_id=6000000173392980834 — he gives five manuscript citations, mostly from the Peniarth collection at the National Library of Wales. That’s what I’m asking about — what manuscripts are Mortimer using? The information should be somewhere, but I don’t have access to the book.
My guess here is that the Anglo-Norman concern with legitimacy — which the Welsh did not share — caused them to assume, since Gwladys was so well placed, that she was Joan’s daughter. But if that information is not showing up in the Welsh manuscripts, it’s not as strong as what Bartrum is seeing — which is that her mother isn’t listed.
There is definitive proof either way, as far as I have been able to find, though certainly if there is we will be glad to shift Gwladys on over!
I note your argument that Joan would not have left a any of her lands to Gwladys unless she were her biological daughter, but Joan, who had come from a French background into her father’s Anglo-Norman court, embraced her role as a Welsh wife — she was called Siwan there, and she was living in a place where her illegitimacy under Church law was irrelevant, and a place where women’s lives differed from what she had known. i would love to know all of her bequests; several of Lewelyn’s children by other women were around, with varying roles in the country.