This shows a bug in hydration where server content is incorrectly being deleted and warned about.

Quoting @sebmarkbage on the cause:

So it happens because React doesn't stop rendering siblings when something suspends. Hydration or otherwise.

So when the sibling suspends, we see that ok we can't commit this. But let's see what happens next. But we don't know how many DOM nodes to skip over because the thing that suspended could render zero or many DOM nodes. So the hydration pointer is left where it was.

I think there was also some related bug with Fragments but I don't know what the repro is.