Interesting search problem… I would probably do a full factory reset too… After you back up your phone, if you have an Apple Watch, make sure you unpair that first before you “Erase All Content and Settings.” Unpairing forces the Apple Watch to make a backup as well. Otherwise this process is fairly painless as long as you set aside some time.
Previously, the issue resolved when I updated to a new iOS version.
But in the last two days I was having sporadic issues, specifically when searching inside Settings. And I’d noticed that my battery once again seemed to be discharging faster throughout the day, suggesting perhaps that iOS was once again constantly reindexing (and failing).
This morning I force-quit all apps, turned off the iPhone XS Max (press volume up button, then press volume down button, then press and hold the power button in quick succession, until “slide to power off” menu appeared), waited a couple of minutes, then restarted. As of now everything seems to work again. Fingers crossed.