Same problem here but don't know why. My device is a95x f3 air with unrooted slimbox 11.5 version.I have one box running 10.4. in the beginning only just after booting the box up did the screen have the white border flash two or three times. But lately after the recent Google software update from the Play store it's doing what the op said, while watching a movie in an app it'll flash a few times randomly I have no idea what is triggering it or what's causing it , but it's happening a half dozen a dozen times a day now while watching the box where it only used to happen just after booting up. Something changed , most likely it's in the Google updates that have come out in the last few months there's been three different updates of webview, Google, Android TV remote, Android TV core ...
I would like to know what causes it also, I have turned on afrd again but that don't change in the middle of a movie and my TV displays the change that's not happening with the white flash boarder.
I haven't met with same issue before, until last weekend. I don't remember that I've changed any settings which might related recently. And before this happened, I've been already using Viper for a couple months. I thought that it might not have business with Viper.
Do anyone still face the same problem now? Are there any other possible solutions? I'm not very expecting but still hoping someone could help for fixing it.