My iPhone is not brand new (it's an X), but certainly not old enough to present any problems.
My battery is at a healthy 90% after a couple almost a couple years of use (bought in Jan 2018 if I'm not wrong).
And not long ago it was even more (like 93%).
But now when I go to battery, it tells me it needs service and that is has "significantly degraded". I've seen this is somewhat common with batteries that are under 80%.
Should I replace mine? I've contacted apple and they told me to schedule sometime in a store.