Why do those buttons flicker?

Code is doing the following:

1. Update title
2. Show them

            if letterButtons.count == letterBits.count {
                for i in 0..<letterButtons.count {
                    letterButtons[i].setTitle(letterBits[i], for: .normal)
                }
            }

            for button in letterButtons {
                button.isHidden = false
            }