<rdar://problem/102050205>

Putting here the steps to reproduce and test what is happening. 

This is the function removing the blockquote

function(t, e, n) {
    var r = n(17);
    t.exports = function(t) {
        return r.write(function() {
            t && t.parentNode && t.parentNode.removeChild(t)
        })
    }
}

In https://platform.twitter.com/widgets.js

To test it. 
1. Put a Subtree Modified breakpoint on <div class="wp-block-embed__wrapper"> 
2. Reload the page
3. Then once it breaks, go back to the Element tab
4. Add a Node Removed on blockquote
5. Disable the Subtree Modified from the parents
6. Continue
7. It breaks on the function above. t being the blockquote