It’s been happening to me as well on an 11 Pro Max for the last month or so on 13.5 and 13.5.1. It happens probably 50% of the time when I’m in a dark area.
I highly doubt this is a hardware issue. It didn’t happen from October (when I got the phone) to late May. No drops, no heavy impacts, and it only started recently once I updated to 13.5. I’m not panicking about it, it’ll be fixed at the latest by iOS 14 in the fall. This is 100% a software issue, so I wouldn’t advise anyone going to Apple to get a display replacement, it won’t do anything to fix it.