Part of mesh is linked to a slider it shouldnt be

Hi. For the exception of female breast bones, weights have no effect on how meshes deform during body customisation. That's uv_1 map's task. When you shorten or lengthen your mesh, make sure to edit its uv_1 map,too

I made a video about it, but it covers mostly uv_1 transfer
There are two ways you can go about it:
Enable sync and edit both 3d model and its uvs
in 3d view X - horizontal axis, Z - vertical, Y - like, depth
in UV editor X - horizontal axis, Y - vertical
For example, here I enabled sync, selected 2 bottom rows, pressed G Z 0,01 in 3d view, then pressed G Y 0,01 in UV Editor

Or you can edit the mesh and then roughly adjust uvs
- edit mesh
- unfold rig and make top visible and selectable;
- select top, make its wireframe visible;
- choose its uv_1 map, enable sync, select everything with A, set Base texture as a background, Shift-select your mesh, select its uv_1 map, select everything, choose THE SAME image as a background, View - Draw other objects