Why does tooltip appear without bootstrap styles in React?

Question

Bootstrap

Here is a piece of code where the tooltip should work

<span className="ml-3" data-toggle="tooltip" data-placement="right"
                  title="Tooltip">
              <i className="fa fa-question-circle-o"></i>
            </span>

But when hovering over the element, the tooltip appears without any styling (just like a normal title image), there are no errors or warnings in the console.
How can this be fixed?
PS I know about react-bootstrap and its components, but because it is not used in the project, I don’t really want to download the library for the sake of one place.
Thanks for any help!