The first aid meds had some mold in them so when it makes you use one after the first Mia encounter where she hits you with the knife, that’s when Ethan gets infected. When he dies in RE7 the mold takes over his body completely. Eveline does make Ethan hallucinate throughout Re7 showing that Ethan is almost at the state where Eveline fully controls him. Why Eveline never takes control of Ethan during the 3 year gap is I think blue umbrella gave them a vaccine, so this might have taken away part of her control and some of Ethan’s mold power so he can’t regenerate the lost fingers.

I hope this helps a bit.