Animator Blend Tree box hides parameter

1. What happened
The parameter slider on the Blend Tree box is hidden by second Blend Tree box

2. How we can reproduce it using the example you attached
Create an Animator Controller
Create 2 parameters (ex: A & B)
Create a Blend Tree (first level)
Create 2 child Blend Trees (second level)
Create another Blend Tree child of top/second level Blend Tree (third level)
"A" parameter should be assigned to all Blend Trees
Now assign "B" to the last Blend Tree (third level)

UI BUG: "B" param slider on top/second level Blend Tree box is hidden when bottom/second level Blend Tree is selected (see attached ParamUnder.png & ParamOver.png images)