setTabBarHidden(_: animated:) didn't show the tabBar

When I called the setTabBarHidden method and passed in false, I found in the KVO callback of the tabBar.isHidden property that the value of newValue was true. Why are the values of the two different? Is this a bug?

Here is the debug stack.

setTabBarHidden(_: animated:) didn't show the tabBar
 
 
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