My guess is yes, some app is staying awake.
It would also explain slow charging....something is eating up CPU cycles and preventing the phone from idling down and sleeping.
It isnt just a 6P thing either....my LG G2 with numerous ROMs/kernels has occasionally experienced issues like this. Almost 100% of the time it could be traced to an app that was misbehaving. You can use a battery tracking app (GSM battery monitor, better battery stats, etc) to get more detailed info. If you arent rooted Im not sure how detailed the info can get, but its probably a lot better than the stock battery graph.
I've just installed BBS, so I'll see if it offers any explanation. If not, I'll probably have to troubleshoot with Safe Mode first, then try to figure out which app is causing this issue. Looks like it'll take some trial and error.