I’m like you regarding working on a project using AMS.
I have a similar issue but regarding CC/Subs, but it answers your question as to why is the CC controls exposed in AVPlayer webvtt-subtitlescaptions-in-a-hls-stream-for-ios-i.html
Here is Apples Doc
_index.html
You’ll see that AVPlayer defaults to showing the CC control when the EXT-X-MEDIA header isn’t present in the manifest.
I have a question for you, how are you avoiding using AVPlayer in Safari on iPhone? I assume you’re using Azure Media Player rather than video.js or such like?