I had this problem too and took my time staging it. Try doing the factory reset again, and reflash with iBasso’s stock Oreo but don’t add on Lurker0’s L1.16 patch. This stopped my random shutdown.
my device is purely stock firmware Oreo, once I encounter random shutdown, I did reset it. After reset, notice that, when the battery hit less than 10%, it will drop about 1% per 10 seconds.