Tip calculator with vanilla JS

This one had me scratching my head for an answer for a good while.

While it is working and it is responsive, to the best of my knowledge, there is a big UX problem due to the action trigger for the math calculations happening on click on the tip buttons and not when the user inserts the input value for the number of people, which I think it was the idea.

As always, I'm welcome to suggestions and helpful hints on how to improve my solution.