Bit of a weird one for you guys. Can't seem to find anything on the web with an answer.
So, this issue has persisted for a long time now. I would say at least a couple months.
Often when I'm scrolling through Tumblr, for example, and I come across an image or video that's a majority black pixels, my phone will just automatically assume something is up and lock itself. It'll then take a few taps of the lock button to bring it back to life - so it's not like I've just tapped the lock button and locked it that way. This also happens when I'm viewing websites, like the one for Vue cinemas, which is mainly black and locks my phone CONSTANTLY when trying to scroll through it.
It's a very specific issue, I know, but I've whittled it down to this one cause and can't find anyone having the same issue.