May as well give the battery health a check. I doubt your phone is too degraded, but on the off chance it got below 80% then phones start acting up.

When the battery gets degraded to the point where it cannot sustain peak performance, iOS throttles the phone to prevent unexpected shutdowns. Maybe your battery is too degraded and you might have turned off the throttling which is causing the phone to shut down when under load.

Have you updated it yet?

Are these restarts happening during the day or overnight? Are you using the iOS beta?

Because sometimes they tend to act weird when there is an update