First the term "leaky gut" is a false idea because every single mammalian gut on this planet is leaky by nature, it is meant to be that way. It is the degree of permeability that matters, anything above the ideal ie Increased Digestive Permeability causes problems for the immune system and liver which can affect other areas of the body by the effect of immune system effects such as antibodies and inflammatory processes. What causes it is not lectins, it is infections by usually biofilm producing bacteria which attack one location.
Just about everyone has these infections, Science and researchers deem them as normal, but they are not normal, they are extremely common. I will not go into what they do to the human body, that is for me to know and everyone else to find out. However they underlie just about every human illness and disease process from hay fever, which is not caused by pollen but a adverse reaction to pollen, to cancer and everything in between including all autoimmune illnesses and inflammatory processes.
Your vegan diet, high in antioxidants and avoidance of proteins similar to human proteins are why you are free of it, for now! That is not a cure if you cannot eat a more inflammatory inducing diet. It is a work around because the infections are still present. That is why a good diet can somewhat offset the damage of IDP but it does not offset all the effects, particularly on the liver function.