My friend who is 70 and has gardened all his life has told me that he has, for years grown pepper plants for himself and chili for his friend in his greenhouse.
One year the friend told him that the chili had no heat in it. He tasted it (hates hot foods) and said he had to agree that it was heatless.
His conclusion was that it was due to cross pollination.
I grow both and last year I thought that the chili I grew was either milder or it was due to me eating so much of it, my taste buds had grown accustomed to the heat.
Now I am not sure. The fruits look perfectly normal, the only change, it seems is the heat of the chili.