Same with my 6s. Beta 2 was ok battery life but still some issues. Beta 3 has been great. My phone has been in standby for 4 hours and streamed music for about 45 min. with the screen off. I'm at 96%.
Im gonna reprise my comment about battery life. The first day of beta three if felt like beta 2 to me but since then battery life has really improved. Much better than 11.4 was. At least for me.
Have seen that message several times. It stops charging but still can sync with iTunes. Happens even when I am simply holding the phone while plugged-in to PC.. not like the cable is getting pulled or something