See https://github.com/whatwg/dom/pull/754 and https://github.com/web-platform-tests/wpt/pull/16348. In particular, you'll need to return early from adoptNode() if the DocumentFragment has a host (and is not a shadow root).
<rdar://problem/57730074>
Unfortunately the initial fix was not quite correct. There's a new fix up at https://github.com/whatwg/dom/pull/819 that I'd appreciate review on.
Pull request: https://github.com/WebKit/WebKit/pull/2034
Committed 252098@main (ab3c0b8cf2a4): <https://commits.webkit.org/252098@main> Reviewed commits have been landed. Closing PR #2034 and removing active labels.