UPDATE: Updating to the latest public beta fixed this. (Could be the device reboot, could be OS beta update. Either way, it cleared the issue for me.) Good luck!

Thanks for the helpful troubleshooting tips - much appreciated! I'll keep digging...

Update 2: Revisiting this for 15.2 and adding the KEYFORMAT="identity" no longer prevents crash on Simulator. Maybe it was just a fluke that it worked before...? Anyway, HLS w/custom url scheme still works on device, just not on Xcode Simulator.

Update: While adding KEYFORMAT="indentity" to the #EXT-X-KEY tag in the m3u8 file did prevent the aforementioned crash on Simulator, strangely, this tag edit now prevents the AVAssetResourceLoaderDelegate from getting called on the device.