My test case uses an audio element created in a different HTMLDocument. When I append it to the current document (with or without adoptNode) the shadow is not created. If I on the other hand do an importNode (which creates a new element) it all works. See URL for test case.
If I understand this correctly, I am able to reproduce this behavior in Safari 15.6 on macOS 12.5 using attached test case in URL field. In Safari, there is only one "Media" element or player while all other browsers (Chrome Canary 106 and Firefox Nightly 105) show three similar to JS showing three audio sources.
I am not sure but I think this is what this bug is for, it could be duplicate of some other bug explaining that Safari is not able to handle multiple audio sources or not. But just wanted to share latest testing results. Thanks!
Reverse duping this to the bug 222657 since it has more context.
*** This bug has been marked as a duplicate of bug 222657 ***