Update - I had one day (I forget which) where the battery drained down normally (which strangely was closer to 8pm when it hit the normal battery level where it dies), but all others had the same issue. Timing has been consistent (between 5-6pm, excluding the day mentioned) and yesterday the phone just shut off in the middle of streaming music with the SiriusXM app to my bluetooth speaker at 5:59 with the screen being off for probably 15 minutes prior. I plugged in the device, which registered 0% for a few minutes, then jumped up to 48%, which seems correct as it had close to 50% when I put the music on.
So we have AT&T, Verizon and Sprint customers experiencing the same issue, so if it's software related it's not carrier specific. I can't speak for other carriers, but Sprint pushed out a 5.1.1 update (version N910PVPU4COJ6 shows on my phone) earlier this month and I never experienced the issue before that.