We had water ingress from the door panel membrane seal following some paintwork, albeit on a newer variant.
The black, bitumous sealant was flattened and the membrane was creased in some areas causing water to pass through, down to the bottom of the plastic door panel then onto and over the rubber door seal.
I tried to pull out the creases and reshape it. It seems to have resolved it.
On an older car, the membrane had been cut for door trim access, again from previous paintwork but that time it was replaced with a whole new membrane. The new one was that foam type material often used to wrap electrical appliances rathe than a plastic sheet.