iPhone Audio Routing Bug: Headphone Switching Issue

I made a video demonstrating the issue: https://www.youtube.com/watch?v=ZYwZWkysls8

Wired headphones (Shure SE215) are connected to iPhone.

0:05 - Call Audio Routing set to "Automatic"

0:18 - phone call initiated, audio goes to Headphones

0:29- tapping the audio button to switch to Speaker

0:35 - tap audio button to switch back to Headphones

0:46 - Call Audio Routing set to "Speaker"

0:56 - phone call initiated, audio goes to Speaker

1:07- tapping the audio button to Headphones does not work, audio is locked on Speaker

1:43 - repeat demonstration

This seems like a bug to me. It is inconsistent behaviour of the Phone app's audio button. If routing was set to "Automatic" the audio button worked. If routing was set to "Speaker" the audio button showed the desired option (to switch to Headphones) but selecting this visible option did not result in an audio change as it did in the other mode.

If Headphones is not an option based on mode, then the interface should reflect this. When the interface shows an option and selecting said option does not work, then the interface is confusing.

Not only is there a visual inconsistency of seeing options and not being able to select them, but there also is a logical inconsistency of a button not offering the user a consistent set of options based on some arbitrary situation (the Call Audio Routing setting). Why should one phone call offer toggle-able Headphone/Speaker functionality and not the other phone call? There's an expectation in the user that if the toggling was there before, it should always be there. There's nothing in the interface interactions that would make the user expect otherwise. User push button, button change audio. So simple yet so elusive.

This behaviour has been consistent across multiple iPhone restarts, including restarts after updating the iOS software, which is currently up to date at 16.4.1 (a).