[UMA2] Mesh problem when head of humanoid avatar was deleted

Hi,

I generated an humanoid avatar using UMA2. When I deleted "Head" of the avatar in hierarchy tap, head bone was deleted well.

But some mesh problem was happened on the edge of neck such as attached file (problem.png). When I checked it out using 3D viewer, the head bone was removed and only the mesh remains, which looks drooping on the floor.

How can I remove all part of head not only bone but also mesh?

(I use unity 2020.3.26f1 version, UMA2 2.12 version)

Thank you!