According to the Wikipedia summary of Phantom Thread:
Alma responds by making Reynolds a poisoned omelette. As he chews his first bite, she informs him that she wants him weak and vulnerable with only her to take care of him. Reynolds swallows the omelette and tells her to kiss him before he is sick.
Reynolds knew that Alma was putting mushrooms in his food which is why she said that she wants him weak and vulnerable.
How did he find out about that and why didn't he refuse the food even though he had a chance to do so? (Could it be something like tired of trying, depression or he can get the mothering he longs for, with the fantasy that he has still come out on top?)