It is not like that in the novel. He knows she is Bezriche's daughter but doesn't believe she is his (Bezriche said dragons only can give birth to a child of someone they truly love --> When she was cursed and dying, she decided to tell Diamid she never liked him (to hurt him so he won't suffer from her death, which is probably the most stupid thind in all the story) --> Diamid casts magic on himself to close his emotions as he was too hurt (he was quite a softie) --> Logically from his knowledge there can't be any kid of him with Bezriche.
The magic that restraints his emotions began to collapse bit by bit. It will be gone when he will save Jayna from allergy (he will cut his hair, which practically contains his power that makes him almost immortal and with that power he'll stop her from dying).
Why did he suddenly become open to the possibility of her being hai daughter Hes been such a hardass abt it and its weird for him to switch up suddenly without anything major really happening or many interactions between them. Like this is the first time theyve really interacted and hes suddenly considering it…