UIStackView not updating after updating height of subview (UITableView with no scroll) while inside a ScrollView

First, a warning:

What you're trying to achieve is not really standard iOS behavior. You should first consider a different approach like creating a single grouped table view with multiple sections. You can implement custom views inside your table view as section headers or footers.

Now if you really wanna go with your original approach...

... for some important reason you should be aware that a table view doesn't have an intrinsic content size by default. Thus, you need to tell the table view how tall it should be because otherwise it will only shrink down to a zero height.

You can achieve this by either subclassing UITableView and overriding its intrinsicContentSize() as Rob suggests in this answer to a similar question.

Or you add a height constraint to each of your table views and set their constants dynamically in code. A quick example:

1. Add both your table views to a vertical stack view in Interface Builder.
2. Give both table views a leading and a trailing constraint to pin their left and right edges to the stack view.

3. Create outlets for your table views in the respective view controller:

   @IBOutlet weak var tableView1: UITableView!
   @IBOutlet weak var tableView2: UITableView!
   
   @IBOutlet weak var tableView1HeightConstraint: NSLayoutConstraint!
   @IBOutlet weak var tableView2HeightConstraint: NSLayoutConstraint! 
   

4. Override the updateViewConstraints() method of that view controller:

   override func updateViewConstraints() {
       super.updateViewConstraints()
       tableView1HeightConstraint.constant = tableView1.contentSize.height
       tableView2HeightConstraint.constant = tableView2.contentSize.height
   }
   

5. Now whenever the content of any of your table views changes (e.g. when you add or remove rows or change the cell contents) you need to tell your view controller that it needs to update its constraints. Let's say you have a button that adds a cell to tableView1 each time you tap it. You might implement its action like this:

   @IBAction func buttonTappen(sender: AnyObject) {
       // custom method you implement somewhere else in your view controller
       addRowToTableView1DataSource()
       // reload table view with the updated data source
       tableView1.reloadData()
       // triggers an updateViewConstraints() call
       view.setNeedsUpdateConstraints()
   }
   

tl;dr:

A UITableView isn't intended for use without scrolling enabled and thus you always need to explicitly set its height when its contents change - may it be using constraints or by overriding the table view's intrinsic content size.