Mesh paint is for painting the vertices of a mesh, but in part uses physics collision queries to detect hits and search for vertices. (see FMeshPaintGeometryAdapterForSkeletalMeshes::LineTraceComponent) In cases where the collision does not well match the mesh vertices, this can lead to Mesh Paint not being able to find/paint all vertices.
Using the third person template attempt to paint the vertices of the mannequin.
Now modify the mannequin's physics asset with a large capsule fully enclosing the mesh.
Try painting the vertices of the mannequin again.
Expected: Mesh paint behaves the same
Result: Mesh paint can fail to find vertices to paint (Depending on the size of the capsule you may need to lower the radius of the brush to see this effect)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-81521 in the post.