The following options combination :
The tooltip won't show up for those points. It will only appear for "circle","rectangle"..
Hi @bogdan-iuga, thanks for the report.
The issue was fixed and will be out on next release.
Checkout the working demo:

Just a curiosity: How did you make the points hollow on hover?
The screenshot is when applied the new css theme, which will be the upcoming feature for the next release.
You can try this just loading the css file in below location.
And to make hollow is easy. Just set data point element's fill to white and set something like:
node {
fill: #fff;
stroke: red;
storke-width: 1px;
}