My app's (simplified) structure is this:
UITabBarController with one
UINavigationController holding a
UITableViewController as root view controller.
when tapping one of the table view controller cells, I push a regular
UIViewController (lets call it VC) end hiding the bottom tab bar. (using the "Hide bottom bar when pushed" flag)
In storyboard I added a regular
UIView subclass to VC that look like a bottom bar, and I use Auto Layout to pin it to the bottom of the VC view.
when I push VC it takes a second for this view to pin to the bottom, it looks like auto layout pin it to the bottom as if the tab bar is not hidden and after a sec it recognise that the tab bar is hidden and moves it to the real bottom of the view.
I know its not the best explanation, so I added a very simple project to demonstrate the issue.