Xcode UITest sometimes does not find property of XCUIElement

Question

In my UI tests, the frame property of some XCUIElement are found, but not of others.
The accessibility identifiers used below are set in storyboard, and app is initialised in setUp() as XCUIApplication().

Here is the storyboard layout:

The two UI elements used in the test are Text Field and Add Button.

Here is the relevant code:

func test() {
    // given
    let mainViewNavigationBar = app.navigationBars[„NavBar“]
    let navBarHeight = mainViewNavigationBar.frame.size.height
    print("navBarHeight: \(navBarHeight)") // is printed out correctly

    let addShoppingItemTextField = app.textFields["TextField"]
    let textFieldHeight = addShoppingItemTextField.frame.size.height // error breakpoint here
    print("textFieldHeight: \(textFieldHeight)")
}

The test stops at an error breakpoint at the second last line with the following message:

No matches found for Find: Descendants matching type TextField from input {(
    Application, 0x60000019f070, pid: 13114, label: ‚xxx‘
)}

I do not understand why the frame property, which should be defined for all XCUIElement, is found in the first case, but not in the second.

EDIT

Oletha pointed out below, that my constant addShoppingItemTextField is an XCUIElementQuery that should be resolved when I try to read the frame property of the textField.
Indeed, when the program stops at the test error breakpoint and I print its description, I get

Printing description of addShoppingItemTextField:
Query chain:
 →Find: Target Application 0x6080000a6ea0
  ↪︎Find: Descendants matching type TextField
    ↪︎Find: Elements matching predicate '"TextField" IN identifiers'  

But the find fails, although Accessibility is enabled, and the Accessibility Identifier is set to TextField:

I also inserted in the app

print(textField.accessibilityIdentifier!)

in viewDidLoad(), and it printed out TextField correctly.

As a workaround, I set the test to recording, and tapped the textField. This created code for the access to the textField. I then replaced let addShoppingItemTextField = app.textFields["TextField"] by (the right side was generated by the recording):

let addShoppingItemTextField = app.otherElements.containing(.navigationBar, identifier:"WatchNotOK")
.children(matching: .other).element.children(matching: .other).element
.children(matching: .other).element

And now the code works without errors.

So it seems to me that the query for the accessibility identifier of a textField does not work correctly.

EDIT 2

I give up: Without changing anything in the storyboard, the test now stops with the same test error (No matches found for Find: Elements matching predicate '"WatchNotOK" IN identifiers‘) at the line let navBarHeight = mainViewNavigationBar.frame.size.height. This did work all the time.
This indicates to me that Xcode UI tests are broken.

Answer 1

I contacted Apple, and they found my bug:
The view of my main view controller had its accessibility property set to true. This was wrong; it must be set to false:

The explanation is found in the docs to isAccessibilityElement:

The default value for this property is false unless the receiver is a standard UIKit control, in which case the value is true.
Assistive applications can get information only about objects that are represented by accessibility elements. Therefore, if you implement a custom control or view that should be accessible to users with disabilities, set this property to true. The only exception to this practice is a view that merely serves as a container for other items that should be accessible. Such a view should implement the UIAccessibilityContainer protocol and set this property to false.

As soon as I set accessibility of the main view to false, the UI test succeeded.