I have 2 phones and there is same issue with both of them. Screen changes brightness level although adaptive brightness is off. This issue occurs while changing phone's position or immediately after tapping. Brightness is changed to 1 level only and then it goes back. I've sent phone back to Xperia for repair but they didn't fix the issue. I've tried to restore factory settings and download newest software but it didn't help neither. There is no specific moment for that to happen but I've noticed it's more often when phones are warm (which is almost always after 5 mins of using Xperia phones).
