String property returning different values when used directly or in string interpolation

I've just finished debugging a situation where some of my labels were not displaying any text, despite the string in question definitely containing a value (it was printing the line before). I eventually pinned it down to returning different values depending on the way I accessed the string. Please note I'm not looking for ways to work around this, I am looking for a reason I am missing as to why this behaviour happens.

I had a protocol with the following optional String property, set to default to nil:

protocol SomeProtocol {
    var titleString: String? { get }
}

extension SomeProtocol {
    var titleString: String? {
        return nil
    }
}

Which was implemented in a class with a not optional String, with title set elsewhere:

class SomeClass: SomeProtocol {
    var title: String
    var titleString: String {
        return title
    }
}

When attempting to access the value of titleString from a SomeClass object, it always returned nil, regardless of the title property. This was seen through assigning to a label like:

label.text = someClassInstance.titleString

However, when I printed the value or set the label through

label.text = "\(someClassInstance.titleString)"

everything worked, and it displayed the text.

I've narrowed the source of this down to where I've overridden the optional property with one not optional. Clearly when I access the property directly it is returned by the protocol implementation, while using it by string interpolation returns the class one. What is actually behind this behaviour?

Edit: to demonstrate this behaviour run this gist in an Xcode playground: https://gist.github.com/CaileanWilkinson/357c17f36d04b522b9bcf1241a825d9f