There is an issue that occasionally occurs when the user is hovering on a node which has a empty string('"") as content, it copy's the tooltip from the previous hovered node. I have added a zip with a gif inside it which shows this.
I have forked your own stackblitz with modification to show this issue:
Reproduce steps yourself using the stackblitz below ->
1. Hover from node1, node3 to node 2.
2. If correctly the tooltip should be empty as always, but after a couple of mouse swiping/hovering node2 has the tooltip of the previous one.
Repeat a couple of times and it appears
Angular 7+, TypeScript 3+