Note: Even with the final version of iOS 14, the slider would not show the Value you set in the storyboard – at least for me. It would always default to showing the slider set to the minimum value. This appears to be a bug that Apple has not fixed yet. It is possible that this will be fixed by the time you read this, but if it is not, don’t worry – you haven’t done anything wrong. It’s just a bug on Apple’s part :]
Perhaps you missed that? Or is the issue you mention something different?