Joe Rossignol, MacRumors:

The Financial Times today highlighted a limited number of reports from users experiencing delays with incoming phone calls on the iPhone X. Apple later confirmed to MacRumors that it is “looking into these reports.”

And:

The report, which links to a few discussions on the Apple Support Communities, notes that hundreds of customers have complained that the iPhone X’s display experiences delays in turning on for up to 10 seconds during incoming phone calls, preventing these users from tapping the Accept or Decline buttons.

Sounds like a very small pool of people have this issue, but it also sounds like this has been around a while (here’s a Reddit comment on this from two months ago) and is still not solved.