@maxmp it didn't take long for the battery drain to happen, even after disabling Chromecast support and restarting my phone. I have attached a screenshot of the battery usage over a ~2.5 hour period after updating back to the latest version.
After restarting my phone it was at about 95% battery remaining. I listened to music for ~30 minutes after rebooting, which you can see with the first slight drop in battery. I believe the drain was more or less normal during this period. I think didn't listen to music for a while, which you can see with the flat line where there was next to no battery usage. Afterward I listened to music for another 40 or so minutes as I finished up at work, then another 20 minutes on my drive home. (it was playing through an aux cable in the car) During this time the battery drained very fast, as the graph shows, and the phone was hot. After getting home, I went for a 30 minute walk, during which I did not listen to music or use my phone at all. (apart from a couple glances to see the state of the battery) The phone was still hot at first, though it did eventually cool down. While the battery usage was less than the massive drain earlier, it continued to drain batter even when not otherwise in use, as compared to the flat line earlier when it had entered deep sleep. Based on the slope of the graph, the drain when not in use during my walk was even greater than the normal drain when it had normal drain during playback earlier.
While the battery usage shows 11% for Poweramp and 1% from Google Play services, since it started at about 95% the total drain was closer to 20%. For reference, during a typical day it will start at 100% in the morning, and between listening to music throughout the day (maybe ~6 hours total playtime) and other light usage I will be ~80% remaining when I get home without any charging in-between.
Based on this, I think we can rule out the Chromecast integration as the culprit. I do not see constant progress bars in the library, and my library is on internal storage (I don't even have an SD card slot on my phone), so I don't think re-scanning is the problem either.