iPhone XS multiple black screen of deaths, with sudden resurrections

My wife's iPhone XS is acting up. It did the following a few times in the past 2-3 weeks: the phone just switched off for no reason (the battery wasn't depleted), and it was not possible to turn it back on. The charge was about half or more. Even if I put it on charger, nothing happened. On a wireless charger, the charger led lit up, so maybe it was charging. It was not hot, so it should be an overheat issue. I also tried plugging in the phone to both a Macbook and a PC, and the phone was not recognized in either, no USB connect sound, nothing. Then, after about half a day, the phone started working again all of a sudden.

The same thing happened multiple times. Black screen, nothing works (I also tried the force reboot keypresses a countless times, to no avail). Sometimes the phone just cames back online after a few hours, sometimes much more. When it comes back online, the charge level is normal... The battery health is 81%, still the original battery.

Any ideas what this could be? Is the motherboard faulty? Or something else? Is it possible to repair this phone economically? (It's a 64GB model).

Thanks!