We likely need to change the logic here slightly, similar to what the BaseNode does (via n7.count != CAPACITY). If we look at BaseNode::InsertChild, the scope of the NodeHandle ends before growing the node. That way, we don't hold on to a node whose memory changes while growing.

{
		NodeHandle<Node4> handle(art, node);
		auto &n = handle.Get();

		if (n.count != CAPACITY) {
			InsertChildInternal(n, byte, child);
			return;
		}
	}
	// The node is full.
	// Grow to Node16.
	auto node4 = node;
	Node16::GrowNode4(art, node, node4);
	Node16::InsertChild(art, node, byte, child);

Thank you for catching that, I will add the changes.

Similarly, we need to scope the NodeHandle here, to avoid holding on to memory that is being compressed as part of the one-way node compression. For reference, see Node4::DeleteChild.

I think freeing will try to grab the same node handle, so we should also add scope here. I.e., we destroy the handle before we destroy the node memory. See Node4::ShrinkNode16.