As just about every map builder is aware, the walkmesh helper is… less than helpful sometimes.
On a hunch, I’ve made some changes and an overriding helper (this one is stone). If you could give it a try.
change details: I made some changes to the helper’s coordinates and walkmesh geometry so it is close to the transition from a stock balcony placeable to the ground, since balcony/ground seems to work better. The helper is slightly higher and the dropoff geometry is wider, close to the balcony connection to the ground rather than an almost vertical geometry of the stock object.