I discovered that the test for this was wrong and fixed it in https://github.com/web-platform-tests/wpt/pull/38581. This essentially asks for a revert of bug 229303 which I suspect was based on this test and not the specification.
<rdar://problem/105631453>
Pull request: https://github.com/WebKit/WebKit/pull/10351
Committed 260539@main (37c7b03669eb): <https://commits.webkit.org/260539@main> Reviewed commits have been landed. Closing PR #10351 and removing active labels.