Xcode UITest sometimes does not find property of XCUIElement

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.