My Lumia 950 also does this. It's the most annoying bug I've so far encountered in Windows Mobile.
Quite often my phone screen remains black after a phone call, when I take the phone from my ear the screen won't wake up, making it impossible to end the call. It also takes forever to wake up and the screen remains dark.
I've tried soft resets, hard resets and throwing the goddam device across the room....nothing helps.
In desperation I've now downloaded the software recovery tool and reinstalled a fresh version of W10M.
Although I was only on the "safe" preview version of the insider program, I've made the decision to step away from it altogether. I'd rather be a couple of updates behind and have a useable phone.
Also (and this may not apply to your situation) my SD card kept showing an error every time I restarted my phone. Someone here mentioned that a faulty sd card can cause operating system issues.
It's been 24 hours and so far my phone seems to be behaving.