On occasion, you may find that VoiceOver focus will jump to the first UI element on the screen soon after you go to that screen, but after you have placed VoiceOver focus on another UI element.
For example, after using View from the Actions menu on a notification and then placing focus on to the Delete button, focus will jump to the Dismiss button. If the timing is right, this may happen just as you double-tap on what you expect to be Delete. Similar behavior can routinely be reproduced after selecting to ‘react’ to a message in the Messages app, where VoiceOver focus will initially be on the ‘Heart’ tapback, but then jump to the Back button.
These are just two examples. Similar behavior is present system-wide.