I wanted to share an experience from last year, my first three layers.
I had to cut them lat summer because time was running up, they had not much roots at all, they were regular AP from a tall tree, I was probably layering 4m above soil level.
They did not bridge over but barely produced roots, they have about 20 leaves each and are doing ok in their first spring.
This might be related to what 0soyoung and smoke are talking about, or that I cut too deep in the sapwood and the leaves were red (autumn color) almost all summer and maybe produced less roots, I do not know.