Missing parameter slider

1. What happened
One of the parameter sliders disappear after adding a new Blend Tree

2. How we can reproduce it using the example you attached
Create an Animator Controller
Create 3 parameters (A, B & C)
Create 3 Blend Trees, each one child of the other
Assign A param to the first BT
Assign B param to the second BT
Assign C param to the third BT
Create one more BT child of the first BT...

BUG: B parameter slider no longer appears on the first BT (there is a slider crack, but no button on it)