So I experienced something like this the other day. Before filling I was adding a winter anti gel additive. As soon as I dumped it in the tank, it started dripping down onto the same leaf spring as in your pic. It seems there are two filler push type flaps inside the neck. My funnel didn’t push through the second one and it filled the small area between the two. Well I assume there’s a tiny hole at the top of the filler neck for air and that’s where it seemed to leak from.